Abstract: A finite element model of retained mandrel service process in 340MPM (Multi-Stand Pipe Mill) was built to research the temperature field change of the mandrel. The thermal stress of the mandrel was also studied to analyze the initiation mechanism and propagation law of fatigue cracks in the mandrel. A comparison between the measured data and the simulation results indicates that the model can reflect the temperature change of the mandrel. After the mandrel was detached from the tube, the highest temperature of the mandrel surface went up to 630℃. Then the mandrel went through three times of temperature-falling in water-cooling stages and three times of temperature-rising in air-cooling stages. At the end of the whole cooling process, the highest temperature of the mandrel surface was 98℃. Both the axial compressive stress and circumferential compressive stress reached to 900 MPa after the mandrel was detached from the tube. At the end of the 3rd water-cooling stage, the axial stress reached to 186 MPa, while the circumferential stress reached to 221 MPa. Cyclic thermal stress could lead to crack initiation and propagation in the mandrel, yet circumferential cracks would be significantly blocked at the place of 17. 5 mm away from the surface, while axial cracks would be significantly blocked at the place of 20 mm away from the surface. The impetus for axial cracks which came from thermal stress was stronger than that for circumferential cracks.