Abstract: In the process of gear hot machining, it is found that there is a phenomenon of coarse and uneven grain, that is, mixed grain, which seriously affects the comprehensive performance of large wind power gear and the popularization and application of the new material. In order to refine the grain and improve the comprehensive mechanical properties of large gear, the austenite nucleation and nucleation growth behavior of 18Cr2Ni4WA large gear steel during post forging heat treatment were studied. The results show that the nucleation and growth of the steel are associated with α phase. It is mainly needle nucleation at the critical temperature(700 ℃), and spherical nucleation occurs above the critical temperature(740 ℃).As α phase recrystallization is completed, the growth tends to be equiaxed, and the austenite is homogenized.