Abstract: Aiming at the phase current imbalance fault of the auxiliary power supply system that occurred during Xiamen Metro Line 1 operation, by analyzing from the angles of grid connected power supply logic and diagnostic functions of auxiliary power supply system, the real cause of the phase current imbalance is obtained. Results show that the medium voltage bus line vehicle-end connector needle is the root cause of the auxiliary power supply system phase current imbalance fault, and optimization scheme is proposed, in which the auxiliary power supply system is divided into two vehicle units to control grid connection respectively.