Abstract: Cellular automaton model based on thermodynamic and evolution kinetic analysis was employed to simulate β to α phase transformation upon welding cooling in the heat affected zone (HAZ) of TA15 alloy. In this model,the effect of cooling rate on the phase transformation undercooling,solution solubility, nucleation rate as well as anisotropic interface moving mobility is considered,which is essential for the welding rapid cooling process. Both the nucleation and growth process were quantitatively characterized. The results show that the α/β interface moving velocity and final microstructure morphology depend both on the intrinsic mobility and aluminum atoms diffusion. α phase plates grow fastest at certain temperature among phase transformation temperature range but finer at lower temperature. Finer α phase plates result from shorter solute diffusion distance rather than larger nucleation rate. As the cooling rate increases, α phase plates grow faster due to larger undercooling but the final transformation percent decreases due to shorter transformation time. Considering the relationship between process,microstructure and mechanical properties,the simulation results indicates that medium heat input is the best welding parameter.