Abstract: Based on the CCT curves and the relationship between different cooling rates and expansion, the phase transformation behavior of 65Mn hot rolled strip during cooling and the variation law between cooling rate and expansion were simulated. The results show that the room temperature microstructure of 65Mn steel is composed of ferrite and pearlite when the cooling rate is 0 ℃·s^-1-10 ℃·s^-1, and the ferrite content gradually decreases with the increase of cooling rate. In addition, with the increase of cooling rate, the expansion caused by phase transformation gradually increases, but in the actual cooling process, the expansion affected by cooling-induced shrinkage is not linear with the cooling rate: the expansion caused by the small cooling rate range of 0.5 ℃·s^-1-2.0 ℃·s^-1 gradually decreases, the expansion at the cooling rate of 2 ℃·s^-1-5 ℃·s^-1 gradually increases, and the expansion gradually decreases when the cooling rate increases to 5 ℃·s^-1-10 ℃·s^-1.