Abstract: Based on the simulation of welding thermal cycle test and fatigue crack propagation test, the fatigue life on coarse grain heat affect zone of high strength steel Q960E, which acted as dynamic load structure, were investigated. The approximate linear relationship between fatigue crack propagation threshold value (ΔK_th) by Paris equation under different welding thermal simulation technique and fatigue life under different cyclic loading were obtained. The crystallographic orientation analysis and expansion mechanism of the crack tip in the fatigue crack propagation specimen were discussed by using the electron backscatter diffraction (EBSD) in scanning electric mirror. The results showed that if stress amplitude ΔP was fixed, the fatigue life N increased with the growth of ΔK_th. Its working micro-mechanism of prolonging life was due to the orientation differences of substructure, resulting in the formation of large Angle grain boundaries (≥ 15°), which effectively forced the crack to swerve, so as to improve the fatigue life of materials.