Abstract: The discoloration reason of the cladding tube of the electrical heating element of a nuclear power plant regulator was analyzed. The results show that uniform and dense protective oxide film was formed on the surface of stainless steel cladding tube exposed to high temperature for a long time, and the oxide film particles were fine. The grain size of different parts of the discoloration cladding tube material was 6. 5~7 grade, and there was no abnormal grain growth, and there was no abnormal content of non-metallic inclusions and α phase. No cracks were observed in the weld between the cladding tube and the end plug.