The invention relates to a CMMC start control method which is a clamp double sub-module modular multilevel converter start method. Sub-modules can be charged to nominal voltage through auxiliary charging, and a whole current positive bridge arm and part of half-bridge sub-module units of a current negative bridge arm are cut. Half-bridge sub-module units, required to be cut, of the current positive bridge arm can be selected with a soft start voltage-sharing strategy. According to the CMMC start control method, various drive states and output voltage of a clamp double sub-module are explained, an appropriate driving state is chosen, and specific implementation of a soft start voltage-sharing process is completed, so that soft start voltage-sharing is easy to implement, and the operation is simple and convenient.