Compared with other detection method, harmonic detection method based on adaptive noise cancellation principle has the advantages like being easier to achieve, strong robustness and so on. This method uses the principle of adaptive interference cancellation in signal processing, takes the grid voltage signal as the reference input and the load current as the original input, eliminates the fundamental active current waveform that is similar to the voltage waveform in the real time from the load current, gets the sum of harmonic and fundamental reactive current, injects a compensation current that has the same value and the reverse polar with the sum of harmonic and fundamental reactive current to load current through compensation device of APF, and then achieve the purpose to suppress harmonic and fundamental reactive current. This paper proposes a new harmonic detection method based on LMS algorithm of improved adaptive gain. What this new method takes as feedback is different from previous researches. Previous studies use the error as feedback, but this new research's feedback is the change form of the error which is the true signal to reflect the system's tracking situation, and this feedback is used to the iterative process of the adaptive filter's weight coefficient. This new method is proved through the MATLAB simulation that it is able to keep small steady state error also has faster dynamic response speed.