Abstract: The simplified thermal transfer model was supposed. The moving coordinate system following the heat source was established by using software ANSYS. The method of step-by-step loading was applied in the simulation. The transient temperature field in different positions and thermal cycle curves at different times of 3 mm LY12 plate were calculated during the whole welding course. So the temperature field distribution in space and time was obtained. The calculated results were validated by measuring the actual temperature of characteristic positions.