Abstract: The nucleation and early growth stage of short crack are important parts of the evolution process and has significant influences on its evolution behavior and life prediction. For microstructural short cracks at the weld toes, taking the microstructure-sensitive fatigue index parameter as the main driving force, the grain model of the weld toe area was built based on the Voronoi method and the early evolution process of the microstructure short crack was simulated. By comparing with the fatigue test results, the rationality and validity of the microstructure -sensitive fatigue index parameter and its calculation models were confirmed. The simulation results showed that the fatigue life was affected by grain locations, grain sizes and grain orientations at the same time. The randomness of grain orientations played an important role in the dispersion of the macroscopic crack depth.