By using FEA software-ANSYS, distribution of 3D temperature field in laser welding for aluminum alloy with different thickness was simulated. In order to improve solution accuracy and efficiency, the transition mesh was used. In view of the characters of laser welding, a travel heat source combined with Gauss function was designed by analyzing both the temperature relativity of the thermal physical parameters of material latent heat of fusion and the effect of convection radiation and characteristics of different thickness on temperature field. By using high-temperature thermocouple, the temperature field was measured. It is shown that the simulation results are in accordance with the experimental results.