The pole-to-ground fault is the most common fault of DC lines in modular multilevel converter based high voltage direct current (MMC-HVDC) system, It is of great engineering significance to analyze the fault characteristics and master the fault current level for the design of relay protection and the optimization of related parameters. As the basis of analysis, the fault current expression of MMC on its DC side is firstly analyzed in this paper, then in view of the MMC-HVDC system with the unilateral grounding metal loop, the coupling effect of the system on the ground resistance on both sides of the fault point is analyzed. According to the causes and essence of coupling, the equivalent decoupling method is proposed, and a practical calculation method for pole-to-ground fault current is obtained. The correctness of the calculation method is verified by comparing with the electromagnetic transient simulation results under PSCAD/EMTDC.