The invention discloses an RH deep decarbonization method for molten steel. The RH deep decarbonization method includes the following steps that a1), an argon blowing pipe supplying CO2-Ar mixed gas is arranged on an RH vacuum treatment device; a2), a steel ladle containing the molten steel is inserted in the RH vacuum treatment device, a vacuum pump is started, when the vacuum of the RH vacuum treatment device is pumped to be smaller than the first pressure, the CO2-Ar mixed gas begins to be blown, and the molten steel is cycled; a3), when the vacuum of the RH vacuum treatment device is pumped to be smaller than the second pressure through the vacuum pump, supplying of the CO2-Ar mixed gas begins to be stopped, and the argon blowing pipe begins to supply first gas so that the molten steelcan be cycled; and a4), when the vacuum of the RH vacuum treatment device is pumped to be smaller than the third pressure through the vacuum pump, the first gas is supplied continuously for 4 min to6 min. By means of the RH deep decarbonization method for molten steel, when an anaerobic gun sprays gas, the end point carbon content can be 13 ppm to 17 ppm or 12 ppm to 14 ppm, and the smelting time is shortened to 2 min to 3 min.