With the help of thermo-calc software, the phase transition and precipitation behavior of Fe- (20~23)Cr-(8~11) Mn-(3~6)Ni-(0.3~0.6)N-(0.3~0.7)C-(0~0.4)Si multi-component system of 21-4N valve steel was studied. TCFE9 database in thermo-calc was used to calculate the vertical section of the system, and the influence of different components on phase transition and precipitation during equilibrium solidification and cooling was analyzed. The precipitation path of phase transition of 21-4N steel was obtained. The results show that the complete equilibrium phase transition path of 21-4N steel during the process of equilibrium cooling from 1 500℃ to 650℃ is as follows: L→L+δ→L+γ+δ→L+γ→L+M7 C3 → γ+M23 C6 →γ+M23 C6 +Cr2 N→γ+M23 C6 +Cr2 N+δ. During solidification, the generation of delta ferrite phase depends on the content of N, C, Ni and Cr in the system. The ferrite phase stabilization zone can be reduced by increasing the content of N, C and Ni, while the ferrite phase stabilization zone can be increased by increasing the content of Cr. The precipitation of M7 C3 phase mainly depends on the content of C in the system. Increasing the content of C and Cr can promote the precipitation of M23 C6 , while Cr can also promote the precipitation of σ phase. N can inhibit the precipitation of M23 C6 and promote the precipitation of Cr2 N.