Basing on the modified SCG model, a high temperature and high pressure constitutive relation of materials is proposed. The change of shear modulus of aluminum with pressure as deduced from this model shows three stages: machining indurations, high temperature inteneration and melting, and turning to zero at the point of complete melting. In the first two stages, the difference in results of modified SCG model and our model is no greater than 4%, which is in the error range of dynamic experiments, and the shear modulus given by this model is in good agreement with experimental data at the point of 145GPa.