In the present study, the stress–strain relationships of Q345B steel with the coupling effect of temperature and strain rate were primarily investigated. First, the quasi-static tensile test and dynamic impact compression test were performed using the 810 material test system and the split Hopkinson pressure bar device, respectively, at temperatures ranging from 25 to 700 °C and the strain rate ranging from 0.001 to 4000 s−1. Second, thermal and strain-rate effects on the mechanical behavior of Q345B steel material steel were studied. It was reported that Q345B steel can be significantly softened with temperatures and hardened with strain rates. Finally, a modified Johnson–Cook (J–C) model was developed to describe the nonlinear mechanical behavior of Q345B steel material over a wide range of strain rates and temperatures.