With the proposal of "double carbon" goal, many new energy sources are connected to the power grid, and the flexible DC transmission system represented by modular multilevel converters (MMC) has been widely used in engineering. Electromagnetic transient simulation plays an essential role in the planning and operation of the MMC system, but the accuracy of the large?step electromechanical transient simulation needs to be improved, and the small?step electromagnetic transient simulation takes too long to meet the development needs of MMC. Therefore, based on the basic idea of the hybrid simulation, the MMC detailed equivalent model is divided into AC and DC parts, the interface equivalent model is established, and the data interaction algorithm is proposed, so as to realize the long?step simulation of AC system and small?step simulation of DC system. The simulation verification of the MMC system based on PSCAD/EMTDC shows that the simulation accuracy of the hybrid simulation method is obviously improved compared with the large?step electromechanical transient simulation, and the simulation time is greatly shortened compared with the small?step electromagnetic transient simulation.