Abstract: The F45MnVS non-quenched and tempered steel was subjected to single-pass compression tests with different deformation amounts (5%-56%) at deformation temperatures of 950-1 050 ℃ and strain rates of 0.01-5 s^-1. The effects of deformation temperature, strain rate and deformation amount on the deformation behavior and grain size of the steel were studied. According to the experimental data, the dynamic recrystallization critical strain model and the average grain size model were established, and the average size of dynamic recrystallization grains was simulated with Deform software embedded with the two models. The results show that with increasing deformation amount or strain rate, or decreasing deformation temperature, the average grain size of the test steel decreased. At relatively high strain rates, the decrease in stress by work softening was not obvious, and the dynamic recrystallization degree was relatively low; the opposite was true at relatively low strain rates. The average recrystallization grain size obtained by the simulation was in good agreement with the experimental results, and the variation of the average grain size with the deformation temperature, strain rate and deformation amount was consistent with the experimental results.