The invention provides a modular multilevel converter for middle-high voltage direct-current grid connection of a high-power sodium-sulfur battery. The modular multilevel converter comprises n sub-unit modules, wherein n is at least two; input ends of the n sub-unit modules all are connected to the sodium-sulfur battery and output ends are connected to a direct-current power grid; each sub-unit module comprises a DC/DC converter, a voltage stabilizing capacitor and an SM unit; each SM unit is a full-bridge power module comprising switch tubes; the input ends of the DC/DC converters are connected to the sodium-sulfur battery; and the output ends are sequentially connected with the voltage stabilizing capacitors and the SM units in parallel. Compared with the prior art, the modular multilevel converter for the middle-high voltage direct-current grid connection of the high-power sodium-sulfur battery provided by the invention improves the overall efficiency of a sodium-sulfur battery energy storage generating system. By a modular design, the redundancy and the reliability of the system can also be improved; and a feasible technical scheme is provided for stable and efficient connection of the sodium-sulfur battery energy storage generating system into the middle-high voltage direct-current power grid.