Abstract: If a mandrel bar is bended too much it will cause rolling passes deteriorated and wall thickness of hot rolled pipes nonuniform. Locations of the maximum and minimum values of wall thickness variations at different cross sections on front end and middle part of pipe body scarcely remain unchanged; but the amount of wall thickness variations gradually increases from tail end to front end. Reducing the residual amount of bending of mandrel bar and the axial force on mandrel bar during rolling will bring about an obvious effect to decrease non-uniform wall thickness of hot rolled pipes.