The equilibrium solidification and cooling phase transition of cast austenitic stainless steel CK3MCuN and the non-equilibrium solidification process in the Scheil-Gulliver cooling mode were investigated via FactSage8.2 thermodynamic calculation software. The results indicate that during the equilibrium solidification process, Fe and Ni are easily isolated by austenitic dendrites, while Cr, Cu, Mo, C and N are easily enriched in the interdendritic region. The interdendritic segregation of Cu gradually weakens, while the segregation of Fe, Cr and Ni gradually increases. In the non-equilibrium solidification process, Fe and Ni undergo negative segregation, while Cr, Mo, Cu, N and C undergo positive segregation, and Cr and Mo undergo very severe segregation in the residual liquid phase at the end of solidification. The main intermetallic phases precipitated during the equilibrium transition process are the sigma phase and Laves phase, with maximum precipitated phase contents of 18.1 wt. % and 12 wt. %, respectively, while the main intermetallic phase precipitated during the non-equilibrium solidification process is the sigma phase, with a maximum precipitated phase content of 0.7 wt. %. In the equilibrium transition process, Mo promotes the formation of the sigma phase and Laves phase, N promotes the precipitation of Cr2 N and inhibits the formation of the sigma phase and M23 C6 carbide, and Cu favors the formation of the ε-Cu phase. In the non-equilibrium solidification process, Mo can promote the formation of the sigma phase and δ-ferrite, N can promote the precipitation of Cr2 N and inhibit the formation of δ-ferrite simultaneously, and Cu can inhibit the formation of δ-ferrite but has little effect on Cr2 N or the sigma phase.