Abstract: The P91 steel pipes and low alloy steel pipes with different hardness values were selected as the research objects to analyze.On the premise of eliminating error caused by environmental factors(such as specimen temperature,rigidity,weight and surface condition),through the hardness comparison test,it was found that the HBHLD value of low hardness P91 steel pipes remained negative deviation compared to the real HBW value.According to the results of the mechanical properties testing,the intrinsic reasons of HBHLD value deviation was analyzed combining Leeb hardness testing principle and the reinforcement theory.The results show that the decrease of hardness of P91 steel pipe accompanied by weakening of solid solution strengthening and dislocation strengthening effects resulted in the decrease of the yield ratio,which was the main reason for the negative deviation of the HBHLD,and the tiny change of elastic modulus had little effect on the deviation.