The high-voltage DC grid based on modular multilevel converters (MMC-HVDC-Grid) is an important direction for the development of power grid in the future. However, it faces great challenges resulting from the serious overcurrent after DC short circuit faults. The pole-to-pole short-circuit fault (PTPSCF) is the most serious fault type of DC fault; its fault current calculation is of great significance to fault judgment, parameter optimization and protection equipment selection. According to the connection modes of converter stations, the topologies of MMC-HVDC-Grid with single voltage level are divided into four groups: ring topology, meshed topology, star topology and hybrid topology. In order to calculate the PTPSCF current under different topologies, the coupling characteristics between converter stations under different topologies after PTPSCF are analyzed, and different splitting methods for different topologies are proposed. Aiming at the situation that a converter station discharges rapidly to both sides of the fault point at the same time in meshed MMC-HVDC-Grid, this paper presents a decoupling method based on electrical distance. Finally, by comparing with the electromagnetic transient simulation results, the efficiency and accuracy of the calculation method in this paper are verified.