The invention discloses a power module dynamic grouping voltage balancing control method of a modular multi-level converter. The method is based on the grouping thought, different groupings are achieved according to the direct current voltage fixed-point bit characteristic in each computing period, power modules are switched, and the purpose of module capacitance voltage balancing is achieved. According to the method, difference between the fixed-point bits of the direct current voltages of all the power modules on each bridge arm of each phase is utilized, dynamic grouping and ranking are carried out according to the number of the power units which need switching and are output through the control algorithm and the current direction of the current bridge arm, and action is carried out on the power units needing to be put into or exit according to the ranking result. The fixed-point bit characteristic of the direct current voltages is introduced in the dynamic grouping and ranking, the advantage of total ranking is achieved, the computation speed is increased greatly, and the large positive role is played in improving the module voltage fluctuation.