Abstract: To meet challenges of GMAW horizontal welding in deep water pipeline laying, such as molten pool falling, narrow gap groove and without backing support, a 3D model of pipe is established and meshed, temperature field material database of API X65 is generated by material software,the Goldak double ellipsoid heat source model is checked by matching the simulated temperature field cloud chart with the actual weld cross section, and temperature field distributions in pipe welding simulated by SYSWELD software are consistent with the actual welding process. The welding temperature field test system is established, and the thermal cycle curve of API X65 pipeline steel plate welding is measured by the thermocouple embedded in the back hole method. Tested curve has the same temperature rise and fall trend as the SYSWELD simulated curve, and temperature of upper plate node is significantly lower than that of lower plate node. The average thermal cycle curve of weld cross section is used as the heat source to simulate the multi pass welding, and the test is carried out. The results show that the welding deformation of multi pass welding is determined by the welding process parameters and residual stress release.