Active power filter (APF) can effectively suppress harmonic current, and harmonic detection is an important part which can directly influence the effect of APF. Based on the traditional adaptive noise cancellation theory (ANCT) harmonic detection method, this paper proposes an improved variable step size adaptive harmonic detection algorithm. This algorithm uses sliding integrator to find the tracking error which can truly reflect the tracking situation, and brings it into the updated formula which is based on the L2 norm to adjust step. The L2 norm refers to the 1/2 power of the sum of squares of the vector elements. In this way, with the improvement of convergence speed and steady-state accuracy, it will have better practicability. Simulation results show that this algorithm can detect harmonic current quickly and accurately. And applying this algorithm to APF can make the effect of harmonic compensation better and enhance the power quality.