Abstract: The shaft of B water-feeding fore pump of a power unit fractured after operation only for 19h.Thefracture reasons of the pump shaft were studied by means of macro examination,chemical compositions analysis,room-temperature mechanical properties test,metallographic examination and fracture SEM analysis,and preventivemeasures were put forward.The results show that the fracture of the pump shaft was fatigue fracture.The fatiguecracks generated from the surface incontinuity and inclusions where stress concentration produced because of variablecross-section,when the pump shaft working under the action of flexural and torsional stress.In the meantime,thestrength of the pump shaft was reduced greatly because of lots of inclusions and freeα-ferrite.So fatigue cracksspread quickly in low cyclic stress condition and the pump shaft fractured finally.