Because the ip-iq harmonic detection algorithm based on instantaneous reactive theory cannot detection reactive current under nonideal power grid voltage, this paper presents the operation mechanism of the shunt active filter with a new control algorithm. The control algorithm is composed of indirect current loop for active and reactive power control and predictive current loop for harmonic filtering control. Due to the parameters of inductor change, the value along with the current will produce error, so a decoupling controller based on synthetic vector is put forward to overcome the defect and improve the anti-interference ability of the system. The simulation is verified to the control strategy by Matlab/Simulink. The results show the correctness and effectiveness.