The invention discloses a modulation method for increasing output level numbers of a full bridge modular multilevel converter (FBMMC). An upper bridge arm and a lower bridge arm of each phase of the FBMMC respectively comprises a full bridge full voltage sub module and a full bridge half voltage sub module, and the AC-side level numbers output from the FBMMC are increased by appropriate switching of the two sub modules. By the modulation method, the AC output level numbers are increased, the effect of a nearest level approximation method is remarkably improved, and the harmonic component of an output voltage of the FBMMC is remarkably reduced; and moreover, by the modulation method, the problems of capacitance voltage deviation of the sub modules, inductance voltage peak of the bridge arms and voltage fluctuation of a DC side of the FBMMC and the like are prevented.