Abstract: Closing failure of high-speed circuit breaker is occurring frequently in Ningbo Metro Line 1 train. The control logic and principle of high-speed circuit breaker is elaborated and root cause of high-speed circuit closing failure is analyzed. It is found that the main contact of K130 is blackened, and the resistance of main contact becomes larger and unstable. Investigation shows that the closing duration of K130 main contact is less than the coil electrified persistent duration of high-speed circuit breaker. K130 contact is closed frequently in unstable current and the vicious cycle after multiple closing causes higher electrical arc, therefore the main contact is blackened during the process. It is proposed to increase K130 main contact closing duration in networks system control software.