The invention relates to submodule triggering methods for a high-voltage direct-current transmission system of a modular multilevel converter. The methods comprise a triggering method for a startup stage and a triggering method for a stable operation stage, wherein the triggering method for the startup stage adopts an auxiliary power supply assisted separate excitation type startup triggering method or an alternating current self-excitation startup triggering method; the triggering method for the stable operation stage comprises steps as follows: direct current energy-storage capacitor voltage of submodules is subjected to capacitor voltage sorting, and capacitor voltage sequences and corresponding sequence voltage values are obtained; meanwhile, a ratio of a bridge arm voltage modulating wave to a direct current energy-storage capacitor voltage rated value is rounded, and the number of the to-be-conducted submodules in a bridge arm at any time is obtained; a sort algorithm based capacitor voltage balance control direct method is adopted to perform capacitor voltage amplitude sorting on the capacitor voltage sequences and corresponding sequence voltage values, and triggering signals of the to-be-conducted submodules are generated according to the flowing direction of bridge arm current; IGBTs (insulated gate bipolar translators) in the submodules are subjected to gating according to IGBT gating signal modules of the submodules.