Abstract: The expansion curves of 29CrMo steel at different cooling rates were measured by thermal simulation testing machine. At the same time, the microstructure and hardness of 29CrMo steel at different cooling rates were analyzed by metallographic examination and hardness test. The results show that when the cooling rate was 0.1-2 °C/s, with the increase of cooling rate, the proportion of ferrite and pearlite in the microstructure decreased gradually, and the proportion of bainite increased. When the cooling rate was 2-20 °C/s, with the further increase of the cooling rate, the proportion of bainite in the microstructure decreased and the proportion of martensite increased. At the same time, the hardness increased with the increase of cooling rate. When the cooling rate was greater than 10 °C/s, the hardness change tended to be stable. The critical cooling rate of martensitic transformation was 10-15 °C/s.