Abstract: In order to establish a simulation method for the fatigue crack initiation behavior of the weld toe, ABAQUS was developed based on the Roe-Siegmund cyclic cohesive zone model to form the VUMAT subroutine for reflecting the fatigue cumulative damage. The cohesive parameters of Q345 weld zone materials were obtained by referring to the literature and experimental data. The Voronoi diagram method and cohesive element method were used to generate a microscopic model with fatigue cumulative damage characteristics and grain characteristics. This model was then merged with the macroscopic butt weld model to simulate multi-scale fatigue crack initiation. The results show that this method can spontaneously select the crack initiation location and the short crack extension path in accordance with the actual situation, and complete the simulation of the microfracture process of the material in the welded area, the critical cycle number obtained by different simulation groups exists in a certain distribution range, and the cumulative cohesive length needs to be fitted by experimental data.