Abstract: In order to solve problem of fault diagnosis of IGBT open-circuit of three-phase four-switch active power filter system, a fault diagnosis method of IGBT open-circuit based on voltage detection was put forward. This method only need to sample a line voltage signal, and combines with current and IGBT switching signals detected by system controllers, then it can quickly and accurately diagnose failure and determine the fault location. Meanwhile, the diagnosis method is suitable for IGBT open-circuit fault diagnosis of single tube and multi-ube. The simulation and experimental results verify the validity and reliability of the diagnosis method.