MMC-HVDC is the core technology to absorb renewable clean energy and construct DC power grid. Transmission line short-circuit fault is a common fault in DC power grid, and its fault current level affects the parameter design of key equipment in DC power grid. Taking Zhangbei DC power grid as the research object, by means of network open-loop, healthy pole open circuit, independence of grounding branch and Thevenin equivalent, the order of the circuit decreases gradually. A decoupled single port simplified model is established. Proposed a single terminal approximate analytical method for DC power system short circuit fault. The proposed calculation method has been verified on PSCAD / EMTDC simulation platform, which has reference significance for fault current influence factor analysis and fault current level suppression.