Abstract: From microalloying and post forging heat treatment optimization, multiple experiments on the grain size stability of 20CrMnMo steel under pseudo carburizing conditions (940℃/36 hours) were conducted in this paper. The experimental results show that, on the basis of containing a certain amount of N, the grain size stability of the test material with only Al added is significantly better than that of the test material with both Nb and Al added simultaneously; By optimizing the post forging heat treatment process, the non-equilibrium microstructure after forging can be improved to a balanced microstructure, eliminating structure heredity, which can effectively refine the grains and improve the grain size stability.