Gas metal arc welding (GMAW) and laser beam welding (LBW) were used to weld 2.8-mm-thick low-alloy high-strength Q345 steel butt joints. The out-of-plane deformation in the welded joints was measured experimentally. Meanwhile, based on the Abaqus software, a thermal-elastic-plastic property-based finite element method considering both material and geometrical nonlinearity is developed to calculate the welding temperature field, welding deformation and residual stress in the LBW and GMAW processes. In the simulation process of temperature field, an ellipsoid volumetric heat source model with uniform density flux was used to simulate the heat input induced by the GMAW process, while a conical heat source model with Gaussian distribution and a combined source model consisting of half ellipsoid volumetric heat source and conical heat source were used to model the heat input induced by the LBW process. The simulated and experimental results show that the out-of-plane deformation induced by the GMAW process is far larger than that produced by LBW. In addition, the area with high longitudinal tensile stress in the LBW joint is much smaller than that in the GMAW joint. Both longitudinal and transverse residual stress distributions in the LBW joint are notably different from those in the GMAW joint. In the simulation of LBW, welding deformation and residual stress distribution in the conical heat source model with Gaussian distribution and in the combined heat source model are similar, although the heat flux distribution in the thickness direction is different. The simulation results are not sensitive to the used heat source models.