The invention provides a unipolar-bipolar convertible direct-current converter comprising a unipolar side power module and a bipolar side power module connected to two sides of a medium-frequency transformer. The unipolar side power module comprises a first power electronic device full-bridge circuit, a voltage stabilizing capacitor and a direct-current breaker; two ends of the power electronic device full-bridge circuit are connected with the medium-frequency transformer; other two ends of the power electronic device full-bridge circuit are connected with a unipolar direct-current system; the bipolar side power module comprises a second power electronic device full-bridge circuit, a capacitor, a voltage stabilizing capacitor, a reactor and a direct-current breaker; two ends of the second power electronic device full-bridge circuit are connected with the medium-frequency transformer, and other two ends of the second power electronic device full-bridge circuit are connected with a bipolar direct-current system. Compared with the prior art, the unipolar-bipolar convertible direct-current converter has the advantages that the circuit structure is compact, fewer switching devices are required, soft switching can be achieved, and system loss is decreased; the unipolar direct-current system and the bipolar direct-current system can be electrically isolated by adjusting windings of the medium-frequency transformer; the converter is also applicable to high transformation ratio applications.