Studying the transient characteristics of DC side faults of a modular multilevel converter (MMC) has important engineering significance for the design of DC system protection configuration and optimization of system parameters. Considering the influence of DC line capacitance on ground, this paper proposes a DC-side single-pole ground fault current calculation method. First, π-type equivalent processing is applied to the DC line, and the generation mechanism of the DC-side single-pole ground fault current in the MMC-HVDC system under three different grounding modes is analyzed. The mathematical modeling of the converter station under different grounding modes is carried out. Based on Kirchhoff's law, the differential equation of the system is established, and the fault current of the system is solved by Matlab. Based on the PSCAD/EMTDC simulation platform, the MMC-HVDC DC system model is established. The simulation results verify the accuracy of the fault current algorithm. This work is supported by National Key Research and Development Program of China (No. 2018YFB0904600).