Abstract: The welding temperature field of the ultrafine grain steel was computer simulated in this paper,and the thermal cycle curve was obtained.The grain size in HAZ was predicted based on the thermal cycle curve.The outcome of the prediction was in accordance with the outcome of the experiment.The heat affected zone(HAZ) is one of the main areas which affect the properties of welds.It is essential meaningful to complete the prediction of grain size by computer using proper methods and that can provide a new and important foundation for optimizing welding parameters and improving the quality of welds.The result showed that:(1) The width of the HAZ was about 6.5 mm;(2) The grain size in HAZ increased obviously and the largest was 180 μm;(3) The higher the cooling speed was,the more refined the grain was,but the refined extent was relatively limited,not obvious.