Abstract: Single-pass isothermal compression tests were carried out on 4Cr5MoSiV1 hot working die steel by Gleeble-3500 thermal simulator. The hot deformation behavior of the steel at deformation temperatures of 750-1 050 ℃ and strain rates of 0.001-0.1 s^-1 was studied, and the microstructures after deformation were observed. On the basis of true stress-true strain curves obtained from the tests, the Arrhenius high-temperature constitutive model at true strain of 0.3 was established. The hot processing maps were drawn by the dynamic material model to obtain the reasonable hot processing window of the steel. The results show that the deformation resistance of 4Cr5MoSiV1 steel decreased significantly with increasing deformation temperature or decreasing strain rate. The hot deformation activation energy of 4Cr5MoSiV1 steel was 594.52 kJ·mol^-1. Within the range of test parameters, the reasonable hot processing window of 4Cr5MoSiV1 steel was at a deformation temperature of 1 050 ℃ and strain rates of 0.001-0.01 s^-1; at this time the carbide was fine and dispersed, and the strengthening effect of the second phase was significant.