Abstract: In order to study the fatigue crack extension mechanism of Q960E SR-CGHAZ of semi-trailer beams in service, this paper establishes a quasi-dynamic stress analysis model and obtains the fatigue stresses by means of dynamic stress test coupled with finite element simulation. The SR-CGHAZ of Q960E was obtained by secondary thermal simulation and the fatigue crack extension rate and impact toughness were tested; the microstructure and M-A morphology were analyzed by LSCM; the substructure and orientation were analyzed by EBSD; and the fatigue crack extension was investigated by TEM. The fatigue crack extension mechanism was investigated by TEM. The results show that the quasi-dynamic simulation model based on dynamic stress measurement has high accuracy; under the condition of peak stress of 300 MPa and stress ratio R of 0.05, the M-A of SR-CGHAZ transforms from point-like to needle-like and short bar-like with the prolongation of the cooling time of t_8/5 in the primary thermal simulation, the impact toughness decreases, and the accompanying fatigue crack extension rate increases. This is mainly related to the broadening of the martensite lath bundles, the change of substructure orientation of the matrix, the reduction of the density of large-angle grain boundaries, and the transformation of M-A into chain precipitated VCr₂C₂-type carbides between the martensite lath bundles.