The high temperature deformation behavior of the 9Ni steel has been studied by the Gleeble-3500 tester. The relationship between deformation resistance and deformation degree, deformation temperature and deformation rate was revealed. The results show that when the deformation degree is less than 0.2, the deformation resistance increases by about 70 to 200 MPa, while the deformation degree varied between 0.2 and 0.4, the deformation resistance increases by about 30-40 MPa, when the deformation degree is larger than 0.4, the deformation resistance increases slowly, some become stable gradually. The influence of deformation temperature on deformation resistance is larger, and deformation resistance at higher temperature is about 160 MPa smaller than at lower temperature. Higher deformation rate leads to larger deformation resistance. The deformation resistance increases about 70 to 110 MPa with the increase of the deformation rate. A new and highly accurate mathematical model of the steel was established to describe the deformation behavior during rolling. The high temperature deformation behavior of the 9Ni steel has been studied by the Gleeble-3500 tester. The relationship between deformation resistance and deformation degree, deformation temperature and deformation rate was revealed. The results show that when the deformation degree is less than 0.2, the deformation resistance increases by about 70 to 200 MPa, while the deformation degree varied between 0.2 and 0.4, the deformation resistance increases by about 30-40 MPa, when the deformation degree is larger than 0.4, the deformation resistance increases slowly, . The influence of deformation temperature on deformation resistance is larger, and deformation resistance at higher temperature is about 160 MPa smaller than at lower temperature. The deformation resistance increases about 70 to 110 MPa with the increase of the deformation rate. A new and highly accurate mathematical model of the stee l was established to describe the deformation behavior during rolling.