As an important part of the modular multilevel converter（MMC），the bridge arm reactor is able to restrain output harmonic current and transient current of converters.The determination of its inductance is crucial to optimizing the operating characteristics of MMC.A bridge arm reactance design method considering restraining the AC side current fluctuation is proposed.The arm reactor is properly simplified based on an analysis of the topological structure and working principle of MMC.The relationship between the equivalent inductance value and the AC side current fluctuation is detailedly derived，which is presented as a fourth-order equation about the equivalent reactance under the condition of a definite amount of the AC side current fluctuation.The equation is solved by MATLAB and the lower limit value of the bridge arm reactance is obtained.The feasibility and veracity of the proposed design method are verified by building a simulation model of21-level MMC in PSCAD/EMTDC.The results of simulation show that the designed bridge arm reactor can effectively reduce the harmonic current，with the calculated values basically in agreement with the experimental ones.