The invention provides a novel modulation strategy of improving output level number of a half-bridge modular multi-level converter (HBMMC). The modulation strategy of improving output level number of a half-bridge type modular multi-level converter is characterized in that the upper and lower legs of each phase of the HBMMC are formed by half-bridge full voltage sub modules and half-bridge half-voltage sub modules; suitable switching of the two kinds of sub modules can increase the alternating current level number AC-output by the HBMMC. The modulation strategy of improving output level number of a half-bridge type modular multi-level converter has the advantages of improving the AC output level number, significantly improving the effect of the nearest level modulation method, and significantly reducing the harmonic wave content of the output voltage of the HBMMC. Moreover, the modulation strategy of improving output level number of a half-bridge type modular multi-level converter does not cause problems, such as capacitance and voltage excursion of sub modules, inductance and voltage spike of legs, and direct current side voltage fluctuation of the HBMMC.