Abstract: Based on lots of low-cycle fatigue experiments of 20 steel at high temperature, surfacial microstructure of the steel was simulated by voronoi-polygen modified by writing Matlab program, and then, the stress and strain state on sample surface under different test conditions were calculated using ANSYS software. The numerical simulation for shor fatigue crack of low carbon steel at high temperature was achieved by the following: using basic energy to characteristic grain boudaries and the ability of slip band resisting crack initiation, and modifying dislocation pile-up theory and then obtain initiation life of crack by calculation. The results show that shor fatigue cracks mainly initiated from persistent slip band and unstable grain boudaries, the cracks exhibited both intergranular and transgranular initiation affected by surfacial microstructure. The modified voronoi-polugen could reflect surfacial microstructure well, and the numerical simulation could accurately reproduce the group initiation behavior of shor fatigue cracks at different cycles.