The modeling of solidification in Continuous Casting demands the mechanical properties of steels at high temperature. These mechanical properties parameters provide the design data for the target surface temperature curve and dynamic control modeling of secondary cooling system, and are used to research hot delivery and crack mechanisms of cast slab. The properties parameters of steels can be obtained by physics simulation experiment. Following processing parameters in Continuous Casting, the stress-strain curves and mechanical properties of steel Q345 were tested and analyzed. Then the curves of relationships between the yield strength, tensile strength, reduction of area, Young’s modulus, plastic modulus of steel Q345 and temperature were obtained. We found the strength of steel Q345 cast slab felled down basically; the ductility was decreased below 700°C, in the range of 725°C-900°C and 1275°C-melting point. The plastic modulus was analyzed with the different temperatures and the different strains in detail. The parameters of mechanical properties for steel Q345 were obtained for using in elastic-plasticity stress model.