There is a contradiction between response speed and detection accuracy in the traditional ip-iq reactive current detection method. In view of this, a variable step size least mean square adaptive filter is proposed to replace the traditional low pass filter. Lorentzian function is introduced to the adaptive filter to adjust the step size. First of all, the characteristics of the step adjustment are analyzed. Then the whole reactive current detection method is analyzed by MATLAB simulation and DSP experiment. The simulation and experimental results show that, compared with the traditional ip-iq method and other kinds of ip-iq method based on variable step size adaptive filter, the operation speed of the proposed method is the fastest, and this method has the characteristics of fast dynamic response and high precision.