The line fault of the modular multilevel converter based high voltage direct current (MMC-HVDC) grid seriously threatens the safe and stable operation of the power system,and the MMC-HVDC grid topology has a significant impact on the fault current. The main flow paths of the discharge current are analyzed when the single-pole grounding fault and the inter-pole short-circuit fault occur in the MMC-HVDC grid with different wiring methods. Then,based on the frequency domain analysis method,it is concluded that the fault current is mainly affected by the topological equivalent inductance parameters,and a simplified index k is proposed to evaluate the topology range and the number of converter stations that significantly affect the fault current. When the single-pole grounding fault and the inter-pole short-circuit fault occur in the power grid,the discharge degree of each converter station in the topology depends on the index k. The larger the k,the greater the discharge degree of the converter station. The smaller the k,the smaller the discharge degree of the converter station. Finally,the five-terminal pseudo-bipolar and five-terminal true bipolar systems are built on the PSCAD simulation platform for verification. The results show that the proposed high-frequency analysis method and simplified index of fault current are correct and effective.