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Due to phase angle measured by phase locked loop has phase difference with grid positive sequence fundamental voltage of traditional id-iq harmonic detection method in grid voltage asymmetry and distortion phase, at the same time, low pass filter performance used in traditional id-iq harmonic detection method will influence accuracy and real-time performance, an improved id-iq harmonic detection algorithm was proposed, namely using of fundamental positive sequence current with good symmetry to instead of instantaneous voltage to lock phase, so as to eliminate measured phase difference of phase angle of power grid, and using of average value theory to replace function of the low pass filter，in order to reduce time delay and improve real-time performance and detection accuracy of the algorithm. The Matlab/Simulink simulation result proves that the algorithm is correctness.