Although Active Power Filter can achieve good filtering effect and accurately compensate harmonic current, it dependents on the proposed current detection algorithm. The paper first discusses the method which is based on transient reactive power theory; the detection method is based on coordinate transformation. After the coordinate transformation is analyzed, the paper advances an harmonic method, this method can remove three-phase to two-phase and two-phase to three-phase coordinate transformation, and effectively reduce the computation, improve the detection speed. At last, the feasibility and validity of the new algorithm have been showed by MATLAB simulation