With the constant expansion of power systems and wide use of many non一linear electronic devices，harmonic pollution of power systems becomes more and more serious，and harmonic is treatment is urgent. The active power filter is a new power electronic assembly connected in the power grid to remove the power harmonic andimprove electric energy of the power grid. This paper aiming at the delay problem by the traditional algorithm，laysemphasis on analyzing the operation principle and structure of the active power filter

proposed on a new harmonicdetection method based on the instantaneous reactive power theory，and carries out simulation model of MATLAB，the simulation results verify this new method which eliminates the delay and the new method is of good compensa-tlOYl effect.