Abstract: THROUGH MACROSCOPIC OBSERVATION, ELECTRON MICROSCOPY ANALYSIS, ENERGY SPECTRUM ANALYSIS, AND METALLOGRAPHIC STRUCTURE ANALYSIS OF THE FRACTURE SURFACE OF THE WELDED JOINT OF THE ION FLAME DETECTOR PROBES, IT WAS FOUND THAT UNDER THE ALTERNATING STRESS OF THERMAL SHOCK AND VIBRATION, MICROCRACKS OCCURRED AT THE STRESS CONCENTRATION AREA AT THE WELD TOE OF THE PROBE TUBE. THESE MICROCRACKS FORMED FATIGUE SOURCES AT THE WELD TOE. AS THE TEST TIME INCREASED, THE CRACKS FURTHER EXPANDED AND EVENTUALLY PENETRATED THE WALL THICKNESS OF THE PROBE TUBE ALONG THE RADIAL DIRECTION AND CONTINUED TO EXPAND ALONG THE CIRCUMFERENTIAL DIRECTION, RESULTING IN FATIGUE FRACTURE. AT THE SAME TIME, SUGGESTIONS WERE MADE TO REPLACE MANUAL WELDING WITH AUTOMATIC ARGON ARC WELDING AND TO INCREASE THE INSPECTION OF THE WELD SURFACE TO ADDRESS THE CAUSE OF FATIGUE FRACTURE.