Abstract: In order to predict the fractures of Q235B and Q345B steel under cyclic loading, the authors used the micromechanical model degeneration of the effective plastic strain model (DSPS) and the cyclic hole expansion model (CVGM) for ultra-low-cycle fatigue fracture prediction to perform the cyclic loading test of notched round bars. The test, combined with the finite element software ABAQUS, calibrates the parameters of the ultra-low-cycle fatigue fracture model for Q235B and Q345B ordinary steel based on the microscopic mechanism. The test samples from Q235B and Q345B base metal, weld deposit metal and heat affected zone were sampled and processed into notched round bars. Ultra-low cycle load tests were performed under two loading conditions, and the fractures were performed using finite element method prediction. The results show that: the Q235B steel exhibits better ductility than that of Q345B steel, but the energy consumption is generally weaker than that of the Q345B steel. The prediction of the CVGM model is more accurate than that of the DSPS model. The prediction accuracy of the microscopic damage model under the C-PTF loading system is good. In the CTF loading system. Both CVGM model and DSPS model can accurately predict the fracture of steel and its welds under cyclic loading.