When two steel plates with different thicknesses are butted together, in order to avoid serious stress concentration caused by the rapid change of the cross-section, the edge of the thicker plate is often gradually thinned to achieve the same thickness at the two edges. The static and fatigue strength of butt joints are higher than other joints. For connections under alternating, shock loads or working in low temperature and high pressure vessels, welding of butt joints is often preferred. The pre-welding preparation of the lap joint is simple, the assembly is convenient, and the welding deformation and residual stress are small, so it is often used in the installation of joints and unimportant structures on site. In general, lap joints are not suitable for working under conditions such as alternating loads, corrosive media, high or low temperatures. The use of T-joints and corner joints is usually due to structural needs. The working characteristics of incomplete fillet welds on T-joints are similar to fillet welds on lap joints. When the weld is perpendicular to the direction of the external force, it becomes a front fillet weld. At this time, the shape of the weld surface will cause different degrees of stress concentration; the stress of the penetration fillet weld is similar to that of the butt joint.