Abstract: With the comparative test rolling operations of the three-roll pipe mill with different deformation amounts, the elements responsible for the internal cracking of the material are analyzed, including the tangential and axial shearing stresses produced during the three-roll rolling process and the tangential and axial tensile stresses produced during plastic bending deformation process. The experiment result shows that with all the other process conditions remaining unchanged, when the deformation amount by the three-roll rolling exceeds a certain value, the internal metal cracking on the inner surface of the tube takes place. Following the main causes of the longitudinal cracking on the inner surface of the medium-wall steel tube, relevant actions are proposed.