Abstract: The working principle and current control method of flexible DC converter were introduced. The influence of current control of flexible DC converter on the electrical characteristics of the AC side was analyzed. The electric quantity change and characteristics of the AC side power grid during the AC side short circuit fault and recovery process were studied. The influence of the AC grid side fault process on the DC grid voltage and the converter output power was analyzed. The four-terminal flexible DC grid simulation model based on the half-bridge modular multilevel converter was built, and the fault process of the AC grid side of the converter was simulated and verified.