Abstract: A model for the thermal process in three-wire tandem submerged arc welding of grade X80 pipeline steel is developed by using finite element analysis software ANSYS. The element birth-death technique is employed to deal with the filling of double-V groove along the longitudinal seam of pipeline steel, and double-ellipsoid volumetric heat source is used to achieve the thermo-load application and solution. The calculated results show that the phenomenon, three-wire in a single weld pool, emerges at 1.6 s after the weld starting, the quasi-steady state is achieved at 6 s, and then a weld pool with length of over 100 mm is produced. The three peak values of temperature at the thermal cycles in weld and heat affected zone elongate the high temperature dwelling time of the thermal cycles, which is the main reason why the grains in the heat affected zone become coarser and the joint properties worsen. The comparison between the experimental and predicted results shows that the agreement extent of the weld width is better than that of the weld penetration, and the calculation accuracy can be improved if the weld reinforcement is considered.