The invention provides an improved structure of a sub-module of a multilevel converter. The structure comprises a first switch module and a second switch interconnected in series, and a negative electrode is connected with a positive electrode of the second switch module. A positive electrode of a first capacitance is connected with a positive electrode of the first switch module, and a negative electrode of the first capacitance is connected with the negative electrode of the second switch module. The negative electrode of the second switch module is connected with the negative electrode of the second switch module, and the third switch module consists of a current-limiting device and a switch device connected in parallel. The switch device consists of an insulated gate bipolar transistor and a diode connected in anti-parallel, and the current-limiting device consists of a capacitance and a diode. The sub-module topological structure is mainly applied to high voltage high-capacity modular multilevel converters, when the side of direct current breaks down, the shut-off of the fault current of direct current is effectively achieved, and breakdown is effectively separated; meanwhile, action of an alternating current breaker is avoided. The sub-module topological structure of the modular multilevel converter is simple in structure and has quick recovery capability from faults.