The invention relates to a sub-module state monitoring method of a modular multi-level converter, which comprises the following steps: respectively monitoring the change situation of a capacitance value in each sub-module in real time, so as to monitor the fault state of each sub-module according to the change situation of the capacitance value in the sub-module. The method for monitoring the capacitance value in the sub-module comprises the following steps: judging the state of charge and discharge of the capacitor in each sampling period; in the charging process of the capacitor, detecting the current of the bridge arm in each sampling period, so that the charging electric quantity of the capacitor is accumulated according to the sampling period and the current of the bridge arm in eachsampling period, and then calculating the capacitance value of the capacitor according to the charging electric quantity, the initial voltage and the termination voltage; and judging the change situation according to the capacitance value of the capacitor calculated many times. The method provided by the invention can comprehensively monitor the fault state of the sub-module, provides auxiliary information for the operation and maintenance of the flexible DC system, improves the pertinence of the maintenance work, and improves the reliability of the flexible DC system.