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In order to predict the elastic constants of 3D four-direction braided composites, unit cell model was established for braided composites with 20°,30°,45° braided angle respectively. The elastic properties of the interior cell and the surface cell were predicted using both the stiffness volume average method and the numerical analysis method, and all quasi-static tensile tests were performed on MTS machine. The experimental results, theoretical analysis and numerical simulation results were analyzed. It is found that the axial tensile modulus decreases,and the transverse tensile modulus and transverse shear stiffness increases with the increase of braiding angles because of the decrease of the component of carbon fiber stiffness in the direction of the carrier movement and the increase of the transverse component of carbon fiber stiffness. Tests reveal that the stress-strain curve for the extension tests of specimen of 20° braided angle exhibit linear elastic type, the stress-strain curve of 30° specimen shows nonlinear characters, the stress-strain curve of 45° specimen shows bilinear relation, but the transverse tensile stress-strain curves of all the specimens are basically linear. Combining the longitudinal and transverse tensile test results with the theoretical and numerical data, it is found that the stiffness volume average method and the numerical analysis methods are able to predict the stiffness in the direction of the carrier movement of 3D braided composites well. However, the prediction precision for the transverse stiffness of specimen is not satisfactory, mainly because these methods neglect the interface damage between the fiber and the matrix in tests.