Abstract: Keyhole gas tungsten arc welding (K-TIG) process of Q345 low alloy steel plates was simulated by SYSWELD software. The temperature field of K-TIG welding process was simulated with three different combined heat sources, and compared with actual weld profile obtained by experiment. It was found that the temperature field obtained by the combination of double ellipsoid heat source on the upper half and 3D Gaussian heat source on the lower half was similar to the actual situation. The effects of plate thickness, gap and welding speed on the deformation and stress of K-TIG welded joints were investigated by K-TIG welding numerical simulation. The results indicate that reducing the thickness of weld plates is beneficial to reducing Z-direction deformation and transverse residual stress, leaving appropriate gap is beneficial to reducing residual stress, increasing welding speed is beneficial to reducing deformation after welding, but is not beneficial to controlling residual stress after welding.