A modular multilevel converter (MMC)-based PV DC boost grid-connection system provides more possibilities for large-capacity grid-connection. However, the transient process of the DC side fault of a PV boosting system is complicated, and the fault current rises rapidly and the peak value is too high. To address this problem, first, the fault process of a DC boost converter and MMC converter station in the case of a bipolar short-circuit fault on the DC side of the system is analyzed, and the fault current flowing through the short-circuit point under reliable blocking is calculated through the fault circuit. Then for the fault current of the MMC converter station, based on its control principle, an active current limiting control strategy based on voltage variation is proposed, one which dynamically changes the reference voltage of the bridge arm by introducing voltage variation to limit the fault current. Finally, the fault analysis results and current limiting effect are verified by a PSCAD simulation model, and it is verified that the control strategy can effectively reduce the opening current of the circuit breaker and the peak overcurrent of the bridge arm.