Based on the hot compress test of 300M steel, the hot deformation behavior and the deformation structure evolution of 300M steel deformed at the strain rate of 10 s-1 were investigated. The results show that when the maximum of reduction of the test sample is 50%, the stress-strain curves of 300M steel are the rheological instability type at the deformation temperature range of 700750 ℃ and the deformation structure appears adiabatic shearing; the stress-strain curves are the discontinuous dynamic recrystallization type with two peaks at the deformation temperature range of 8001000℃, and there are two rounds of dynamic recrystallization during the hot deformation; the stress-strain curves are the discontinuous dynamic recrystallization type with only one peak at the deformation temperature range of 10501180℃, and there is only one round of dynamic recrystallization during the hot deformation.