Ferrite stainless steel has been widely used in foreign automobile vent pipe. During the production of domestic automobile vent pipe, because the crystal grains of welding Joint heat-affected zone are not refined by heat treatment and become very big, forming properties of cold machining of ferrite stainless steel become bad. In order to optimize welding technology, the temperature field of laser welding joint of stainless steel automobile vent pipe is simulated. The temperature field model of laser welding joint of 409 L stainless steel pipe is built based on the pair heat source model. At the same time, the affecting factors are considered, such as thermal radiation, phase change latent heat, thermophysical properties of materials with temperature changes and the spread of convective heat in the liquid metal in the model. The temperature field in different welding parameters are simulated. The impact of process parameters on the temperature distribution is acquired. The experimental results can help works, such as accurately understanding laser welding thermal process, predictive control of welding stress and deformation, and improving cold forming quality of automobile vent pipe.