Abstract: Addressing the problem of the pit defect of the mandrel as occurring during the pipe-rolling process with the mandrel mill，a 3-D thermo-mechanical coupling model is established with the ABAQUS FEA software in a bid to investigate the in-rolling process mandrel force characteristics and the mechanism as affecting the forming of the defect. As a result it is revealed that during the rolling process，obvious stress concentrations present on the mandrel outer surfaces of the groove top and the roll gap transitional zone，and the stress peak of this transitional section higher than those of the groove top and the roll gap by 23.7% and 32.4% separately，and the crack source is easy to develop in the said zone；when the coefficient of friction between the mandrel and the inner surface of the steel pipe is increased from 0.11 to 0.15，the mandrel friction increases by about 40%，while the surface compressive stress increases by 13%；the increase of the mandrel retaining speed just slightly affects the friction，however the mandrel surface compressive stress decreases，and in turn the length of the mandrel working section correspondingly increases. Accordingly improving the lubrication conditions during the production process can effectively reduce the occurrence of the mandrel pit defect so as to prolong the service span of the mandrel，and thus further improve the steel pipe quality. Moreover to properly increase the retaining speed can reduce the stress as the mandrel bears.