The design of the security and protection system is the key technology in the application of the active power filter（APF）. The security and protection system for shunt APF can be divided into three parts， the insulated gate bipolar transistor（IGBT） self-protection technique， APF safeguard measures in abnormal and fault situations as well as APF defend measures in normal situations. A simple and effective start-up process and two output current limiting algorithms are proposed based on an in-depth research on the preventive protection for the three-phase four-wire four-leg shunt APF. Simulation and experimental results demonstrate the effectiveness of these protection measures that enable the APF protection system to have its role of compensation brought into full play while ensuring its own security.