Recently studying technical analysis, I found that many people actually don't have a deep enough understanding of the KDJ indicator. This indicator, known as one of the "Three Treasures of Retail Investors," seems simple, but to use it well, you need to master quite a few details.

First, let's talk about what KDJ is. Its full name is the Stochastic Indicator, and its core function is to help investors identify trends and optimal entry points. On the chart, three lines appear—K value (fast line), D value (slow line), and J line (direction-sensitive line). Among them, the K and D lines are mainly used to judge overbought and oversold conditions, while the J line measures the deviation between the K and D lines. When these lines cross, it often signals a new trading opportunity.

Specifically, the K value measures the relationship between the closing price of the day and the price range over a past period; the D value is a smoothed line of the K line used to eliminate noise; the J line reflects the divergence between the K and D values. Theoretically, when the K line breaks above the D line, it is a bullish signal indicating a buy; when it breaks below, it is a bearish signal suggesting a sell.

In terms of calculation, KDJ is derived by comparing the highest price, lowest price, and closing price within a specific period to get the raw stochastic value (RSV), then calculating K, D, and J values using a smoothed moving average method. The formula is RSV = (Today's closing price - lowest price over n days) ÷ (highest price over n days - lowest price over n days) × 100, with RSV fluctuating between 1 and 100. Then, K and D are calculated, with today's K equal to two-thirds of the previous day's K plus one-third of the current RSV; D and J are similarly recursively calculated. However, in actual operation, most trading platforms have already done the calculations, with parameters usually set to (9,3,3); we just need to adjust them as needed.

In practice, I usually draw two horizontal lines at 80 and 20. When the K and D lines rise above 80, it indicates the stock is entering overbought territory; dropping below 20 indicates oversold. The amplitude of the J line can also be used for judgment—when the J line exceeds 100, it indicates overbought; below 10, oversold.

There are four main methods to judge buy and sell signals. The first is the Golden Cross—when the K line and J line are both below 20, and the K line crosses above the D line from below, forming a low-level golden cross, indicating weakening of the bears and a potential rally, which is a buy signal. The second is the Death Cross—when the K and D lines are both above 80, and the K line crosses below the D line from above, forming a high-level death cross, indicating the bulls are exhausted and bears are starting to reverse, which is a sell signal.

There is also divergence. Top divergence occurs when the price makes higher peaks, but the KDJ values make lower peaks, usually signaling an impending reversal downward, a sell signal. Conversely, bottom divergence occurs when the price makes lower lows, but the KDJ peaks are higher, indicating a bottoming out and rebound, a buy signal.

Additionally, you can observe the top and bottom formations of the KDJ indicator. When the indicator runs below 50, if W-bottoms or triple bottoms appear as reversal patterns, it suggests the market is about to shift from weak to strong, and investors can buy the dip. Conversely, when the indicator runs above 80, if M-top or triple top formations appear, it indicates a potential reversal downward.

I remember the Hong Kong Hang Seng Index in 2016. In early February, the index kept falling, but smart investors noticed that although the price was making lower lows, the KDJ indicator showed a bullish divergence with higher lows. This was a desperate time for ordinary people, but for savvy traders, it was a rare opportunity to build positions. On February 19, the Hang Seng opened with a large bullish candle gaining 965 points, a 5.27% increase. By February 26, the K line broke above the D line from below, forming a low-level golden cross, prompting investors to add positions, and the index then surged another 4.20%. On April 29, a high-level death cross appeared, prompting timely profit-taking. This case vividly demonstrates the power of the KDJ indicator in real trading.

However, the KDJ indicator also has limitations. In extremely strong or weak markets, it can become dull, leading to premature signals and unnecessary losses. Its signals are also lagging, based on historical data, and cannot reflect rapid market changes in real time. Moreover, during sideways or choppy markets, KDJ can generate false signals, misleading traders. Most importantly, the KDJ lacks independence and should not be used as the sole decision-making tool.

Therefore, I recommend combining the KDJ indicator with other technical analysis tools. Using candlestick charts, J line trends, volume, and other indicators together can help more accurately judge market changes and reduce trading risks. Continuous practice and experience are needed to compensate for the indicator’s shortcomings, so that one can succeed in the market.