Abstract: In the process of pre-installation inspection of a P91 steel pipe, the test results of the portable Leeb hardness tester were low. The reasons for the low hardness were analyzed by chemical composition analysis, hardness test, metallographic examination and other methods. The results show that the microstructure and hardness of the steel pipe did not meet the standard requirements and were unqualified products. The curvature and wall thickness of the sample were too small, the support ring was not used during the test, and the hardness conversion method was not correct, resulting in differences in the test results of the portable Leeb hardness tester.