The calculation of fault current is an important link in the design and planning of flexible DC power network and the judgment of fault type. Based on the circuit theory and numerical integration principle, a numerical calculation method of fault current based on the discretization of dynamic components is proposed. First, the circuit theory is used to analyze the fault discharge mechanism to simplify the power network reasonably, then the dynamic components are discretized by the Backward Euler method, and the discrete model of the simplified circuit is established, based on the simplified discrete model of the circuit, an improved nodal equation is established to calculate the fault current at each time. The feasibility and generality of the proposed method are verified by comparing the results of the proposed method with the results of the simulation model of the VSC-HVDC grid built in PSCAD/EMTDC.