A failure analysis was performed on a fractured twin-screw pump shaft. Through the analysis of the macromorphology and microstructure of the fracture, the analysis of the chemical compositions of shaft material, the metallographic test of the specimens cut from the fracture, the investigation of the operating condition of the pump, the main reason for the fracture can be described as the increased rotating and bending load caused by low viscosity medium. Corresponding preventive measures were put forward. (c) 2017 American Institute of Chemical Engineers Process Process Saf Prog 37:95-103, 2018