The mechanical behavior at high temperature of 45CrMo high strength steel was studied by Gleeble-3800 thermal simulation testing machine. The results show that under the deformation temperature of 800-1100 ℃ and the strain rate of 0.01-10 s-1, the true stress increases firstly and then decreases with the increasing of true stain, that is, both strain hardening and strain softening effects appear. The relevant parameters in the Hansel-Spittle constitutive model were determined by fitting the experimental data under different high temperature deformation conditions. Compared the prediction value of Hansel-Spittle constitutive model for high temperature compression performance of 45CrMo high strength steel in the hardening and softening stages with the experiment value, the results show that the average relative error between the predicted and  experiment values is only 0.04% under the same deformation temperature and different strain rates or under the same strain rate and different deformation temperatures, which verifies the validity and accuracy of Hansel-Spittle constitutive model and its parameters. Especially, the Hansel-Spittle constitutive model is suitable to the material which has a strain softening effect during the high temperature deformation.