The present disclosure provides a DC conversion system and a control method thereof. The DC conversion system comprises: an upper power module group, a lower power module group, input terminals of the upper and lower power module group are connected in series, and output terminals of the upper and lower power module groups are connected in parallel; the controller configured to receive an input voltage of respective input terminal of each of the first and second power modules, a first output current of the output terminal of the upper power module group, a second output current of the output terminal of the lower power module group, and a total output signal of the output terminal of the DC conversion system, and generate a modulation signal according to them to control a power switch of the corresponding power module.