A multi-level DC-DC converter device includes an inverter, a 3-winding high-frequency transformer, a first full-bridge rectifier, a second full-bridge rectifier, a selective circuit and a filter circuit. A first winding at a primary side of the high-frequency transformer connects with the inverter while a second winding and a third winding of at a secondary side of the high-frequency transformer connect with the first full-bridge rectifier and the second full-bridge rectifier. The selective circuit connects with DC output ports of the first full-bridge rectifier and the second full-bridge rectifier, thereby operationally selecting two serially-connected full-bridge rectifiers or single full-bridge rectifier to output two voltage levels performed as a multi-level output voltage. The filter circuit connects between the selective circuit and a load for filtering harmonics and outputting a DC voltage.