The 3D thermo-mechanical simulation on two-high cross piercing elongating process of steel 9Cr-l Mo φ485 mm x 73 mm large size heavy wall pipe has been carried out by using commercial element software MSC. SuperForm. The damage field of rolling piece during elongation process and the tendency of inner surface crack of steel pipe are analyzed by using Oyane criterion for ductile fracture. It is found that there is a high dangerous crack band in internal surface of steel pipe adjoining contact zone with plug. The influence of roll entrance cone angle on damage field of rolling piece is studied. Results show that as the roll entrance cone angle increases from 3. 0° to 5. 0° the maximum damage characteristic value decreases from 0. 520 7 to 0. 354 4 and the tendency of inner surface crack reduces.