Objective：To develop and validate an accurate three-dimensional geometrical and mechanical finite element（FE） model of the lumbar L4-L5 segment using a new computer-aided designing （CAD） method, Methods： First, a modified ＂nonseed region segmentation＂ was done to extract the interest region in the CT image and to obtain a binary image, from which the iso-surface of vertebral body was produced by a discretized marching cubes algorithm. Second, ＂best cross-section planes＂ representing the morphologic characteristics of physiological lordosis were used for the initial iso-surface model, forming a ＂nonregular piecewise subspace＇. This subspace and the embedded iso-surface model were subsequently transformed by local affine transforms to a ＂regular subspace＇, in which a surface mesh of high quality was generated quickly. Finally, a reverse transform procedure was employed to restore the original three-dimensional （3D） image of the lumbar surface mesh of lumbar L4-L5. All coordinate dada of nodal points and message of triangular patches of the surface model were then subjected to ANSYS for the three-dimensional FE mesh construction. An accurate 3D non-linear FE model of lumbar motion segment （L4-L5） was developed and validated against published data. Results： The constructed FE model of lumbar L4-L5 consisted of 94 794 solid elements, 1196 link elements, 1170 shell elements, 768 target elements and 464 contact elements, and included geometrical, material and contact non-linearities. The predicted results of lumbar L4-L5 segment were closely correlated with published results of experimental biomechanics in terms of axial displacement, segment rotation and intradiscal pressure under similar load condition. Conelusion： Based on advanced algorithm,this constructed surface model of L4-L5 segment is capable to perform whole digitalized binary image extraction and reconstruction of the lumbar surface with excellent simulation results