The invention discloses a method for vacuum refining of molten steel. The method for vacuum refining of the molten steel comprises the steps that slagging-off treatment is conducted on molten iron after pre-desulfuration treatment is conducted on the molten iron, so that molten iron with the sulfur content being lower than or equal to 0.005% is obtained; dephosphorization treatment and decarburization treatment are conducted on the molten iron, so that first molten steel is obtained; the first molten steel is placed in an RH furnace, the corresponding vacuum degree of the RH furnace is controlled to be 2 KPa-12 KPa, constant-oxygen treatment is conducted on the first molten steel, and a treatment way is determined according to the C content of the first molten steel and the constant-oxygen temperature and the oxygen content which are obtained through the constant-oxygen treatment; and the first molten steel is treated in the treatment way, and then carbon-deoxidization treatment and secondary combustion treatment are conducted, so that second molten steel is obtained. According to the method for vacuum refining of the molten steel, the carbon-deoxidization technique is assisted with the secondary combustion technique to increase the temperature of the molten steel, in this way, production cost can be effectively reduced, the quantity of materials containing aluminum alloy can be reduced when deoxygenation treatment is conducted on the molten steel, the quantity of generated aluminum oxide impurities is greatly reduced, and the purity of the molten steel is improved.