Abstract: The austenite grain growth process of 45 steel in AC flash butt welding heat-affected zone(HAZ)was simulated by genetic algorithm method based on actual measured welding thermal cycle curves,which the energy of the system in minimum state and the grain boundary moved to curvature direction in grain growth process were considered.The average radius actually measured in coarse grain area and fine grain area of welding HAZ are 54.30 μm and 13.58 μm,and agree with the average grain radius of 51.50 μm and 16.29 μm simulated by genetic algorithm simulation method.Genetic algorithm method results show the average radius of grain growth in welding HAZ during welding heat recycles is 66.8 percent of that during the whole heat.