The invention discloses a modular multilevel converter circulating current suppression method, and relates to a flexible direct current transmission field of a power system. A hybrid particle swarm optimization algorithm is adopted to optimize PI parameters of a PI controller, a closed loop control circuit is composed of the PI controller and a controlled object, the PI controller outputs the input of the controlled object, the controlled object outputs the output of the system, the output of the system is sent to the input end of the system to form negative feedback, and is subtracted from the input of the system to generate a system error, and the error is sent to the PI controller. An objective function is constructed by the system error and the output of the PI controller, the hybridparticle swarm optimization algorithm is used for finding optimal PI parameters, the system dynamically adjusts the output and input to optimize the objective function, and when the objective functionis optimal, the parameters obtained through optimizing of the hybrid particle swarm optimization algorithm are the optimal PI parameters; the circulating current suppression method has high circulating current suppression precision, fast response and strong adaptability, can adjust the controller parameters according to the system dynamically, reduces the loss, and improves the overall steady state operation efficiency.