The invention discloses a solid state heat sink device used for friction stir welding and a welding method of the solid state heat sink device. The solid state heat sink device comprises a solid state heat sink, a push rod connected with the solid state heat sink and a cooling water pipe, wherein the solid state heat sink comprises an upper bottom face, a lower bottom face and a side face, heat dissipation through grooves are formed in the lower bottom face, cooling water channels are formed in the side face, the cooling water channels are connected with the cooling water pipe, and an installation hole used for installation of the push rod is formed in the upper bottom face of the solid state heat sink. The solid state heat sink device synchronously moves in the welding direction along with a friction stir welding device, and the lower bottom face makes full contact with the surface of a weldment so that heat conduction can be achieved, heat dissipation and cooling can be conducted on a weld joint which is still at a high temperature, and the peak temperature of the weld joint can be effectively lowered. Meanwhile, cooling water is injected into the water channels formed inside the heat sink to conduct forced heat dissipation and cooling. According to the solid state heat sink device and the welding method, the weld joint can be prevented from being overheated, weld joint crystalline grains are refined, the well-formed formation is improved, residual stress and strain of a welded joint can be eliminated or reduced, and the aims of improving the quality of the weld joint and improving the comprehensive performance of the joint are achieved.