The thermal expansion curves of 34MnB5 steel at different cooling rates were measured by DIL805A/D thermal expansion instrument. Ac1 and Ac3 were measured according to the experimental data. By combining the metallographic method and the hardness method, the austenite continuous cooling transformation curve (CCT curve) of the experimental steel was drawn. The structure transformation and hardness change of the experimental steel at different cooling rates were analyzed. The results showed that: when the cooling rate was slower than 3 ℃/s, the phase transition products mainly included ferrite and pearlite structures, and the average microhardness was 223.7HV; when the cooling rate was in range of 315 ℃/s, the phase transition products mainly included the mixed structures of bainite and martensite; when the cooling rate was fast and higher than 15 ℃/s, the single phase martensite structure was formed, and the average microhardness was 552.7HV.