The embodiment of the invention discloses a buck converter and a voltage reduction method, device and system. The buck converter includes a first voltage reduction unit, a second voltage reduction unit and a direct-current support capacitor unit, wherein the first end of the first voltage reduction unit is connected with a positive-voltage input end; the second end of the first voltage reduction unit and the first end of the second voltage reduction unit are connected with one end of the direct-current support capacitor unit and a positive-voltage output end respectively; the third end of thefirst voltage reduction unit and the second end of the second voltage reduction unit are connected with the other end of the direct-current support capacitor unit and a negative-voltage output end respectively; the third end of the second voltage reduction unit is connected with a negative-voltage input end, so that the voltage between the positive-voltage output end and the negative-voltage output end is smaller than that between the positive-voltage input end and the negative-voltage input end. By the adoption of the buck converter and the voltage reduction method, device and system, the voltage reduction efficiency can be improved, and the cost of the buck converter can be reduced.