The invention provides a modulation strategy of a DC (Direct Current)-DC convertor and a submodule voltage-sharing method thereof. The modulation strategy comprises the following steps: 1) constructing a switching function model of a submodule; 2) determining a ladder undulant bridge arm switching function model; and 3) according to the ladder undulant bridge arm switching function model, modulating the alternating-current side voltage current waveform of the DC-DC convertor. The voltage-sharing method comprises the following steps: constructing an energy model, and changing energy interaction on the submodule by primary side alternating current on the basis of the energy model so as to balance submodule voltage. Compared with the prior art, the modulation strategy of the DC convertor and the submodule voltage-sharing method thereof can improve the intermediate alternating current voltage frequency of the DC-DC convertor on the premise that switching frequency is not improved, reduce capacitance voltage fluctuation, reduce the volume of submodule capacitance in the convertor, are favorable for reducing the volume of passive devices including a bridge arm inductor, a transformer and the like, lower cost and cause the DC-DC convertor to own high power transmission capability and power density.