The key to accurately predict welding residual stress is to explore the solid-phase transition law of SA508-3 steel and establish the phase transition model under continuous cooling conditions. The effects of cooling rate on phase transformation and strain response of SA508-3 steel were investigated in this paper. The established programs accurately predict the evolution of microstructure at different cooling rates, the relationship between the bainite volume fraction and temperature is fitted by a modified bainite transformation model and is validated by optical microscopy and dilatometric test. The strain evolution of SA508-3 steel is predicted at different cooling rates and is verified by the dilatometric test. In addition, the effects of bainite transformation and martensite transformation on the strain response of SA508-3 steel are intensively discussed. This work casts light on the simulation of metallurgical effects at different cooling region during welding.