Abstract: Based on the failed feed water pump shaft in HRSG（heat recovery steam generator） of 9F gas turbine and normal 17-4PH stainless steel, the fracture reasons were analyzed and the difference of alloying element content, microstructure and mechanical property between the failed pump shaft and normal 17-4PH steel were researched using chemical composition analysis, optical microscope, energy dispersive spectrometer（EDS）, scanning electron microscopy（SEM）, high resolution transmission electron microscope（HRTEM） and mechanical property test. SEM analysis of fracture showed that large fluctuation was observed in two fracture surfaces with clearly fan-shaped steps, which present the obvious characteristic of multiple-sources of fatigue and fracture. The chemical composition analysis reflected an extremely low content of alloying element of Cu（lower 3%） and Ni（lower 4%）for the failed pump shaft, which caused the lower mechanical properties caused by the less alloy strengthening phase result from the misapplication of material. The contrastive analysis of tensile test and hardness test show that the failed feed water pump shaft shows lower tensile strength（Rm） and brinell hardness（HBW） than those of normal 17-4PH steel（500MPa and 150HB, respectively）, which are attributed to the finer and homogeneous columnar martensite construction, and the second phase dispersed uniformly in the matrix of normal 17-4PH steel.