As the interphase circulating current exists in the three-phase bridge arm of modular multilevel converter, the differential flatness theory (DFT) based MMC circulating current suppressing controller is deduced. According to the analysis about the nature of the interphase circulating current and the definition of the DFT, it chooses the input, output and intermediate quantity of the control system, proving that the design of circulating current suppressing controller can meet the conditions of DFT. The newly designed controller could reduce the interphase circulating current in the bridge arm current, and make it closely approximate the sine wave. At last, the MMC model is simulated based on Matlab/Simulink. In the case of no impact on MMC external voltage and current, the result show that the control model is effective, which provides a new idea for the design of circulating current suppressing controller.