The invention relates to a capacitance reducing auxiliary circuit based on a modular multilevel converter, and belongs to the technical field of high voltage and ultrahigh voltage DC power transmission. The capacitance reducing auxiliary circuit is formed by three phases of symmetrical capacitance branches and an inherent bridge arm reactor which is matched with the modular multilevel converter soas to realize the effect of reducing the design capacitance value of converter bridge arm submodule capacitors. The capacitance reducing auxiliary circuit is embedded in the connecting part of the upper (lower) arm of the modular multilevel converter and a DC side positive (negative) bus and generates resonance voltage in normal operation to change the bridge arm current harmonic component of theconverter so as to influence the charge and discharge process of each bridge arm submodule capacitor and change the capacitance voltage waveform of each submodule in the energy transmission process.The oscillation amplitude of the submodule capacitance voltage in case of transmitting the same active power can be effectively reduced through adjustment of the capacitance value of each capacitancebranch so that the effect of reducing the design capacitance value of the submodule capacitors under the premise of the same oscillation amplitude requirement of the submodule capacitance voltage canbe achieved.