Modular multi-level converter (MMC) mathematical analysis, including the main circuit parameter design, the main circuit electrical state, controller design and parameter tuning, and so on. From the external characteristics, MMC can be considered that the equivalence is reasonable and effective to some extent; but from the internal characteristics, the kind of equivalent will ignore the MMC some internal system characteristics, so the way modeling cannot reflect the constraints of voltage and current within the system. Therefore, it is necessary to carry on the system modeling research to the MMC converter to reveal the constraint relation of the internal electrical parameters of the MMC converter, and provide the theoretical basis for the system analysis and the complete set design.