The current tracking control is one of issues in controller designs of shunt active power filter（APF）. The idea of passivity is a powerful tool to study the stabilization of the nonlinear system. In this paper， the state space model of the four-leg APF for three-phase four-wire systems is determined firstly. A new passivity-based current tracking dynamic controller is then proposed. Based on the analysis of the working principle of APF， the sufficient conditions to achieve the gradual tracking and to keep the internal stability of the APF control system are given. Simulation results performed in PSCAD verify the validity of the new approach.