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The thermal coupling casing behind high-pressure adjusting steam stigma valve was broken during operation. One casing which has shortest life had worked only three monthes. In this paper, some analysis and calculation had been done. The results showed that there are some reasons which caused the fracture as follows: the interactive effect of integrated fatigue and fretting fatigue led to the failure, periodically pressure fluctuate in pipe behind the casing was the main reason which caused integrated fatigue and fretting fatigue, and the fatigue was accelerated because the nature frequency of this thermal coupling casing was within range of the pressure fluctuate which loaded in this casing.