The invention provides a modularization multi-level converter submodule grouping stair wave modulation method. The method comprises the steps of dividing submodules in a commutation chain into N submodule groups, rounding reference voltages of the submodule groups to obtain rounded reference stair wave voltages Vstair_wave, distributing trigger pulses to submodules inside the submodule groups, and carrying out voltage-sharing among the submodule groups. According to the modularization multi-level converter submodule grouping step wave modulation method, the submodules are divided into the N submodule groups, each submodule group is taken as a controllable voltage source, stair wave modulation is adopted in the submodule groups, the N submodule groups adopt rounding correction, and therefore a purpose similar to carrier phase-shifting is achieved; and due to the fact that voltage stability control is carried out among the submodule groups by means of a proper voltage-sharing control method, difficulty of the stair wave modulation is reduced, and requirements of the modulation algorithm and the voltage-sharing control algorithm for software and hardware are greatly reduced.