Section flattening is an inevitable physical phenomenon in the forming process of tube bending, and severe section flattening will affect the reasonable assembly of the tube fittings, and therefore restricts its wide application. Finite element (FE) model for numerical control (NC) bending of titanium tube considering the variation law of contractile strain ratio (CSR) and the Young"s modulus (E) is established in this paper. The section flattening behaviors of TA18 tube under different geometric conditions and different process conditions have been investigated. The results show that considering the variation law of CSR-E can make the cross-section change, which has no remarkable influence on the change law. The reasonable range of geometric and process parameters are obtained, which provides a basis for studying the forming prediction and controlling the final precision for the NC bending of TA18 tube.