In view of the low-pass filter selection problem based on instantaneous reactive power theory, fundamental wave current of extracted through Fast Fourier Transform analysis, to every harmonic content and total harmonic distortion as selected basis of Butterworth low-pass filter order and cut off frequency. Firstly, established the simulation model of harmonic detection based on the instantaneous reactive power theory in common connection point of harmonic source; Secondly, according to distortion degree of the extraction of fundamental wave current, low-pass filter selection method is described in detail; Finally, introduced open-loop harmonic detection system based on the theory of model reference adaptive, overcame the internal parameter change and external disturbance in practical model for the influence of harmonic detection. Theoretical analysis and simulation experiments show that the system has good harmonics detection precision and fundamental wave extraction is better robust and adaptive.