In this paper, the AC grounding fault characteristics on the valve side of an MMC converter station are studied. It is found that when the metal grounding fault occurs in the AC system on the valve side, the grounding current at the fault point is composed of the capacitor discharge current of the lower bridge arm and the capacitor discharge current of the upper bridge arm and the opposite bridge arm, and the grid side AC system will not feed the grounding current. For a grounding fault with transition resistance, the AC system at the grid side will act on the grounding current at the fault point through the transition resistance. The mathematical expressions of grounding fault current and bridge arm current under a three-phase grounding fault and single-phase grounding fault are derived in detail. The correctness of the expression is verified by RTDS simulation. Since the potential at the valve side AC outlet is clamped to 0, the sub module capacitance of the upper bridge arm will produce an overvoltage. For the discharge capacitor circuit composed of the lower bridge arm and the fault point, the fault current flowing through the lower bridge arm increases rapidly and cannot be cut off through the circuit breaker. Therefore, it is recommended to take reliable current limiting measures to avoid serious damage to primary equipment caused by a valve side fault, such as to the converter station. This work is supported by the National Natural Science Foundation of China (No. 52167011).