Abstract: In order to resolve the fatigue fracture problem of the steel structure, the multi-scale fatigue crack propagation process of the damaged low-carbon steel Q345 is simulated by using the finite element software ABAQUS in which the influence of the micro and meso defects of materials has been considered based on the principle of stress equivalence. The results illustrate that the damage caused by micro and meso defects of materials shows great influence on the distribution of the plastic area, the stress intensity factor and J-integral along the macro-crack tip; especially for the macro-crack with enough length, it would significantly accelerate the propagation of the macro-crack.