The invention relates to an improved harmonic detection method. The improved harmonic detection method comprises the following steps: whether network voltage has distortion is detected; when the distortion is detected, the detected distortion is preprocessed; the voltage is sampled; an obtained square signal which is synchronous with the network voltage is taken as input of a phase detector; a lowpass filter and a function generator constitute a PLL (phase-locked loop) circuit through the phase detector. According to the method, distortion detection of the network voltage and preprocessing are performed through a sensor and a DSP (digital signal processor) before a traditional voltage sampling link for detecting voltage phase information through PLL, so that the problem that fundamental positive-sequence active current cannot be obtained accurately due to the fact that the phase detected by the PLL is not the phase of voltage positive-sequence components when the power voltage has distortion is solved.