Abstract: Leakage reasons of the guide valves for air conditioner when inspecting by 420 MPa air under water were studied by means of stereo microscope, scanning electron microscopy and X-ray energy dispersive spectrometer. The results show that because some parameters of welding technology were not control strictly during the course of automatic argon arc welding, temperature of the local welding zone was higher than the normal temperature, which result in the formation elements segregation and reduction of grain-boundary strength. Then Intergranular welding thermal cracks generated under the action of the welding thermal stress, so air passage of the guide valve formed and leakage happened when it was test by high pressure air under water.