The application of permanent magnet brushless motor in industrial robots is increasing gradually. For the open-circuit fault mode of a power transistor of the brushless motor control system, the analysis was conducted under two different operating conditions, namely, light load torque and heavy load torque. A fault localization and protection method is proposed using the signals of the three-phase hall position sensor, bus current, and three-phase winding current. Finally, a hardware test platform was built, and the basic electrical signals such as communication bus and power transistor drive signal were explained. Then, the software was used to simulate the open-circuit fault modes of six switch transistors one by one. After multiple experiments, it was found that the fault localization accuracy was high and the localization time was short, which could effectively identify and judge the switch transistor faults, and had good practical application value.