Abstract: Considering the effect of adhesive layers between adjacent yarns, a new FEM model of representative unit cell was established, which more really simulates the inner meso-structures of the 3D 4-directional braided composites. The boundary conditions and constrain conditions of the unit cell model were discussed and the effective elastic properties parameters were obtained by FEM calculation. The calculation results by the model agree well with the experimental results. The relations of the effective elastic properties varied with different braiding angles and fiber volume fractions are concluded by analyzing the results. The distribution of the stress in the composites was determined, which provide data for the strength calculations of the composites.