Abstract: To accurately predict the longitudinal tensile mechanical properties of 3D four-directional braided composites, parameter modeling for surface and interior unit-cells mesoscopic solid models was implemented and the deviation of yarn spatial traces and squeeze deformation of yarn cross-section were considered in the surface unit-cell model. Voxel mesh was used to discrete models on which appropriate boundary conditions were imposed and damage models for each constituent of composites were added into a user-defined material subroutine (UMAT) in finite element analysis software ABAQUS. By simulation analysis of surface-interior unit-cells models for 3D four-directional carbon fiber/epoxy braided composites with 30° and 45° interior braiding angles respectively, using the volume-weighted average method, longitudinal tensile modulus and strength for braided composites specimens with different thicknesses were predicted. The progressive damage process of composites was studied by counting the number of integration points with the same damage modes. Results show that the longitudinal tensile mechanical properties predicted based on surface-interior unit-cells models for 3D four-directional braided composites agree well with experimental results, and damage analysis results reflect reasonably progressive damage process of surface and interior unit-cells.