According to the postweld heat treatment (PWHT) for main steam pipe using P92 steel in 1000 MW ultra-supercritical thermal power units, the heat treatment process with the main and auxiliary heaters made of flexible ceramic resistance heating at the same time, measuring the temperature both on inner and outer wall, and plugging both ends is applied, by which the inner wall temperature of welding could be up to 740 ℃ and the minimum temperature difference between inner wall and the outer wall is 15 ℃. The PWHT test was carried out on the main steam pipe using P92 steel in 1000 MW air cooling unit of Ningxia Lingwu Phase II Project. The test results show that, for P92 steel, especial the PWHT of thick pipe, the heat treatment process could make the inner wall temperature of weld seam meet the requirements of tempering temperature of P92 steel isothermal process and ensure the uniformity of mechanical properties of welded joints.