A typical single-phase short-circuit fault was designed and analyzed for the typical valve-side of the MMC-HVDC system. Without considering the asymmetry of the number of sub-module switching in the transient process, this paper established the AC-side equivalent circuit and DC-side equivalent circuit respectively when there is single-phase ground short circuit in the valve side bus bar. Then the transient formulas of the valve side bus voltage, current, DC -side voltage and current were obtained under the fault condition. Besides, power change before and after the failure and its influence on neutral current, input and output, active and reactive power were analyzed. Taking the flexible transmission project of ±20KV/43.2MW in Shanghai as an example, the comparison between MATLAB/Simulink simulation test and the established analytical formula model shows that the transient analysis formula was consistent with the analysis result of circuit simulation model. Finally, fault characteristics of single-phase short-circuit of HVDC system were verified.