Abstract: The weld thermal cycle curve of 45 steel in alternating flash butt welding HAZ was simulated according to the actually measured temperature field of axisymmetric weldments.The relationship of tMCS to the actual time t and temperature T was determined with the parameters in HAZ by MC method.The MC simulation makes the average grain size in the welding HAZ after thermal cycle process as 70% of that after isothermal heating at same tMCS.The welding heating process makes the grain grow only 25%-30% of the final grain size.The simulated results are in agreement with the measured results.