A high temperature alloy pressure conduit in helium mass spectrometer leak detection,the side seam leakage rate,by using chemical composition analysis,metallographic examination and fracture analysis method analyzes the causes of leakage rate exceeds bid. The results showed that the inner surface of the weld was seriously oxidized due to the failure of argon gas protection in the tube during welding,and thermal cracks formed on the outer surface of the weld center during the second welding of the weld. The leakage of the catheter was mainly due to the fact that a large number of oxides on the inner surface of the weld reduce the strength of the weld,and the outer surface of the weld had been polished,forming a stress concentration at the grinding place. Under the effect of the hydraulic air-tightness test and repeated assembly of the catheter,the thermal crack developed plastically and formed a passage through the wall thickness.