Abstract: After using for a period of time, a lot of four-way valves used in air conditioner leaked at weld seam. The leakage causes at weld seam of four-way valve were analyzed by macroscopic analysis, chemical composition analysis, metallographic examination, hardness test, energy spectrum analysis, ion chromatography analysis and simulation test. The results show that the four-way valves were exposed to corrosive medium containing sulphur and chlorine, and blue-green and black corrosion products were produced on the surface of four-way valves. The weld seams were penetrated by corrosion and eventually leaked.