The invention discloses a method for smelting 9 Ni steel by an electric furnace. The method comprises the following processes: an EAF furnace primary smelting process, a VD furnace vacuum residual-oxygen decarbonizing process, an LF furnace refining process and a VD furnace vacuum treatment process, wherein in the VD furnace vacuum residual-oxygen decarbonizing process, a steel ladle enters a VD furnace, vacuum is released when vacuum degree is lower than or equal to 200 Pa, and carbon in molten steel is decarbonized until C content is lower than or equal to 0.01%. The method adopts the EAF furnace primary smelting to control the temperature and components of tapped molten steel, then, enters the VD vacuum decarbonizing process, enters the LF furnace refining process to remove oxygen and regulate components, and finally controls a casting temperature through the VD furnace vacuum treatment process. The produced 9 Ni steel is qualified in components, and is less in impurities; a rolled-out steel plate has a thickness lower than or equal to 50 mm, has yield strength Rp0.2 greater than or equal to 515 MPa, has tensile strength Rm of 690-825 MPa, has lowest impact power greater than orequal to 21 J at a temperature of (-)196 DEG C, and has lateral expansion greater than or equal to 0.381 mm.