Abstract: A new model of dendritic growth based on cellular automaton method was developed to simulate the columnar-to-equiaxed grain transition and the distribution of solute concentration during solidification in Fe-C alloy weld pool. The influences of different disturbance amplitudes and different cooling rates on the columnar-to-equiaxed grain transition were considered. The results indicate that the grains increased gradually and became stable eventually. When the cooling speed increased, the growth rate increased and the microsegregation was more serious, and the columnar-to-equiaxed grain transition happened easier. Meanwhile, the disturbance amplitude affected the dendritic morphology and especially the growth of secondary and tertiary dendrite arms.