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In recent years, Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) transmission system has gradually developed, and the AC short-circuit current contributed by both the converter and AC system needs to be solved urgently. In order to solve this problem, the paper focuses on the transient process of phase current change on the AC side after a unipolar ground fault occurs on the DC side. Then, an equivalent model of the AC system short-circuit current is established. Therefore, a calculation method of the short-circuit current contribution of the AC system based on the arithmetic circuit is given. Finally, a simulation model of two-terminal project built based on PSCAD verifies the correctness of the proposed calculation method.