The invention relates to a method for smelting COST-FB2 steel through gas-phase nitriding under a negative pressure condition. The method comprises the following steps that firstly carbon deep deoxidation is performed under the condition of high vacuum degree, deoxidation is completed after the surface of molten steel is stable and bubbles do not rush out any longer; then an alloy element for promoting nitrogen dissolution is added and completely molten, and then gas-phase nitriding is started; during gas-phase nitriding, nitriding pressure during gas-phase nitriding is calculated by using thermodynamic calculation software Factsage, the molten steel temperature is accurately controlled, and the nitriding time is calculated and determined through the surface area of the molten steel, the volume of the molten steel and the nitrogen balance content; when gas-phase nitriding is about to be completed, the alloy element boron and the volatile manganese are added, then through argon filling,pressurization is performed until the high pressure is reached, pressurization casting is performed under the condition that the nitrogen partial pressure is kept unchanged, the under-pressure stateis kept in the solidification process, vacuum breaking is performed after complete solidification, and a COST-FB2 steel cast ingot which has the content of N being 0.015%-0.03% and the content of O being less than or equal to 0.0035%, is uniform in component and compact in structure is prepared.