The permanent magnet synchronous motor (PMSM) has many advantages, such as high energy density, high efficiency and high power factor, and its application field is rapidly expanding. The traditional main circuit structure of three-phase three-leg topology with the space-voltage vector pulse width modulation (SVPWM) technology can reduce the winding harmonic currents and meanwhile improve the utilization rate of the DC voltage, thus reduce the torque ripple of the motor and broaden the speed range of motor effectively. However, in a-phase fault, the system reliability will be difficult to be maintained by the traditional topology structure. Therefore, it is limited on some occasions where reliability is strictly required, such as aviation, aerospace, marine and explosion-proof, etc. On this basis the paper puts forward a three-phase four-leg control system with the fault-tolerant function. Namely, on the basis of traditional three-phase four-leg control system, the fourth leg is added and connected to the motor neutral point, and three dimensional voltage space vector modulation (3d-SVPWM) technique is utilized to drive PMSM, which make PMSM has a good running characteristics and can guarantee system safe and reliable running in the case of the lacking phase or the one-phased open-circuit fault.