Aiming at the product with variable cross-section structure, a friction stir welding method for variable cross-section lap joint was proposed.By adding an auxiliary plate, the entire weld was compensated to equal thickness in the welding direction.Welding adopts frist-position welding + second-position welding + formal welding process,frist-position welding was pre-positioned, second-position welding ensured an effective connection between the auxiliary plate and the main structure of the test piece,and the formal welding ensured the formation of a complete weld. The surface of the weld is well formed,and the ultrasonic phased array detection has no defects beyond the standard.Through analysis of mechanical properties, the mechanical properties of the weld at the variable cross-section and the non-variable cross-section are basically the same,the average tensile strength reaches more than 70% of that of the base material,and the average elongation is above 5.8%.The morphology of the grains at the weld nugget of the variable cross-section weld is fine equiaxed grains. The grain size near the shoulder impact zone is larger than that near the weld root. The lap interface of the auxiliary plate at the heat engine affected area on both sides of the weld disappears because of recrystallization influence.