Abstract: Tensile properties at 20, 540, 560 ℃ and stress-rupture strength at 540, 560 ℃ of P91 steel specimens with different hardness were tested. On the basis of the approximate parallel nature of stress-rupture strength versus rupture time curves of P91 steel at certain temperatures and hardness, the linear equation of stress-rupture strength and rupture time was established for P91 steel with different hardness. By using this linear equation, the stress-rupture strength versus rupture time curves of whole life cycles were obtained by fitting the short-term stress-rupture data of different hardness specimens, and the fitting results of stress-rupture strength were compared and analyzed with the test results. The results show that the fitted stress-rupture strength had relative errors of less than 6% comparing to the test results, indicating relatively high fitting accuracy. By obtaining the full-life stress-rupture strength versus rupture time curves of P91 steel with different hardness with short-term stress-rupture data, the stress-rupture strength of P91 steel parts with abnormal hardness can be evaluated without destroying the integrity of parts.