Abstract: Q345 steel has been widely used in steel structures at high temperature. Creep fracture failure is the fundamental failure mode of steel structures at high temperature due to the creep crack propagation behavior of Q345 steel with defects. In order to resolve the creep fracture problem for steel structures at high temperature, the creep crack propagation of Q345 steel at 400 ℃ were simulated under three different loading levels. The influence of crack length and loading level on the steady state creep transition time, time-varying integral creep factor and creep factor were studied. Constraint effect along crack tip was studied using C^*-A₂ two-parameter method. The creep crack propagation behavior was analyzed under different working condition. The results show that loading level has great influence on the creep transition time and creep factor during the creep crack propagation of Q345 steel with defects. The constraint level of crack tip gradually increases during crack propagation,which is slightly affected by loading level. The creep crack propagation rate increases gradually during crack propagation, and the loading level has great influence on the creep crack propagation.