Keyhole gas tungsten arc welding (K-TIG) of Q345 low alloy steel plates was simulated by using SYSWELD software. The temperature field of the K-TIG welding process was simulated with three different combined heat sources and was compared with the weld profile that was obtained experimentally. The temperature field that was obtained by a combination of a double ellipsoid heat source on the upper half and a three-dimensional Gauss heat source on the lower half was similar to the real situation. The effects of plate thickness, gap and welding speed on the deformation and stress of the K-TIG welded joints were investigated by K-TIG welding numerical simulation. A reduction in the thickness of the weld plates reduced the z-direction deformation and transverse residual stress; an appropriate gap reduced the residual stress and an increase in the welding speed reduced deformation after welding, but did not help to control the residual stress after welding.