The invention relate to a modular multilevel converter bridge arm equivalent method, which is characterized by comprising the steps of establishing an MMC (Modular Multilevel Converter) bridge arm equivalent model under normal operating conditions, wherein the step includes the sub-steps of establishing an MMC bridge arm equivalent model respectively under a non-locking condition and a pre-charging condition; establishing an MMC bridge arm equivalent model under a DC side fault condition, wherein the step includes the sub-steps of establishing an MMC bridge arm equivalent model respectively before and after the locking of the converter when the DC side breaks down; and combining the MMC bridge arm equivalent model established under the normal operating conditions and the MMC bridge arm equivalent model established under the DC side fault condition so as to establish an MMC bridge arm equivalent model applicable to various operating conditions. The method has the advantages of ability of being scientific and reasonable, simple calculation, easy implementation, ability of being applicable to various operating conditions and high simulation accuracy.