Recently, I’ve seen many new traders asking how to use the KDJ indicator, so I’ll organize my understanding. To be honest, KDJ is indeed something every retail trader must learn, but many people learn the formula without knowing how to apply it in practice, which is a waste of time.

First, let’s talk about what KDJ is. It’s called the stochastic indicator, and the chart has three lines—K line (fast line), D line (slow line), and J line. The relationship among these three lines is quite interesting: K and D lines mainly look at overbought and oversold conditions, while the J line actually measures the deviation between K and D. Simply put, the J line reflects the divergence between K and D, and the greater the difference, the larger the volatility of the J line.

Theoretically, a bullish signal occurs when the K line breaks above the D line, and a bearish signal when it breaks below. But in practice, it’s not that simple. I usually judge with four methods: golden cross, death cross, top divergence, and bottom divergence.

The golden cross is my favorite buying opportunity. When both K and D lines are below 20, and K crosses above D, it’s called a low-level golden cross. I’ve seen many times that right when this signal appears, the stock starts a beautiful rally. Conversely, a death cross occurs when both K and D are above 80, and K crosses below D, which is a typical sell signal indicating a market reversal.

Divergence is where real skill is tested. Top divergence happens when the stock price keeps making new highs, but the KDJ indicator forms lower peaks at high levels, signaling a potential decline. Bottom divergence is the opposite: the stock keeps making new lows, but the KDJ forms higher peaks at low levels, often indicating a rebound is beginning.

There’s also pattern recognition. When KDJ is below 50 and forms a W bottom or triple bottom, it’s a buying opportunity; when it’s above 80 and forms an M top or triple top, it’s time to consider exiting. For example, I’ll use the Hang Seng Index in 2016. On February 12, the index plunged sharply, and everyone was panicking, but I noticed that although the price kept making lower lows, the KDJ was making higher lows—a classic bottom divergence. As a result, on February 19, the index surged with a 5.27% bullish candle, and those who caught this move made a lot of money. Then, on February 26, a low-level golden cross appeared, leading to another rally. By April 29, a death cross formed at a high, so I exited to protect profits. On December 30, a double bottom pattern appeared, signaling another buy-in, and the bull market officially started.

But I have to be honest—KDJ also has obvious drawbacks. It’s too sensitive to trends, often giving early signals, and in very strong or very weak markets, it can become dull. Also, its signals are lagging because they’re based on past data, so it may not react quickly in rapidly changing markets. The biggest pitfall is that it can generate false signals, especially during sideways consolidation, leading to wrong buy or sell prompts.

Therefore, my advice is: never rely solely on KDJ. It should be combined with other technical indicators, candlestick patterns, volume, and multiple dimensions for judgment. KDJ is just a tool to help confirm the trend, not the whole decision-making process. In practice, risk management and mindset are the most important; indicators are just auxiliary. If you’re interested in deepening your trading skills, you can practice on a demo platform. Only through real trading can you truly understand the power of these indicators.