The path planning of fillet welding in the flat position by industrial robot cooperating with positioner was studied.Considering the requirements of fillet welding in the flat position, the evaluation index of robot motion stability performance under the constraint of welding torch attitude was designed and used as the objective function of welding evaluation; the gray wolf optimization algorithm was used to optimize the cooperative welding path, to ensure that all welding points in the welding seam were in the optimal fillet weld in flat position and welding gun was in the ideal welding attitude. The results show that the trajectories of robot and positioner are very smooth in the whole welding process, which can effectively guarantees the welding quality and improves the production efficiency.