Abstract: Based on software SYSWELD, the temperature field of U71Mn rail steel overlay welding was simulated. The influence of preheating, post-heating and preheating + post-heating on the cooling time (t8/5) was studied , and the microstructure of heat effect zone was characterized to verify the correctness of the numerical simulation. The results showed that all the welding processes can prolong the cooling time. The effect of preheating+post-heating process was the best, and post-heating was better than preheating. Preheating + post-heating process was an effective measure to control the hardening structure and cold crack. The calculated process can be used to control the hardening structure, and the simulation results were verified by experiment.