Flexible DC distribution network has flexible control, high power quality and can be equipped with new energy distributed generation, but the serious over-current problem after DC line fault is one of the important factors restricting its development. The analysis and calculation of short-circuit fault current is of great significance for the design and selection of system parameters. However, as MMC converter station is a highly complex nonlinear system, the exact analytical expression of fault current can not be obtained, so the short-circuit fault current is often obtained by simulation. The research on approximate expression of fault current can only be carried out for two terminal DC system, but it can't be done for line short-circuit fault current in other topologies. This paper introduces the basic characteristics and simplified method of modular multilevel converter (MMC).