The calculation of AC fault current plays an important role in the selection of circuit breakers and DC protection, especially in the case of asymmetric faults. However, the calculation of AC fault current for DC protection mostly depends on electromagnetic transient simulation. The method has several drawbacks such as time-consuming simulation, lack of physical mechanism analysis and parameter influence factor analysis. In order to solve the problems above, this paper derives rising rate of the bridge arm current and AC line current before the response of control system, and the analytical models of the d-axis and q-axis positive sequence components of AC line short-circuit current and three-phase short-circuit current after the response of control system for the AC asymmetric fault scenario in the symmetric single pole MMC-HVDC system. The analytical model is compared with the simulation results of an HVDC engineering project to verify the correctness of the fault analysis model. The significant value of this study is that the dynamic process of the short-circuit current under AC asymmetric fault can be obtained by theoretical calculation without multiple electromagnetic transient time domain simulation results, and the final calculation results can provide reference for the design of control and protection strategy.