Abstract: Based on the existing experiment observation and theoretical research, the spatial configuration of interior yarns was analyzed in detail, a new microstructure model of 3D braided composites was established and the mathematical relationship between technics parameters and microstructure model parameters was obtained. The new model truly reflects the jamming pattern between yarns and its cross-section variation along the axial of the yarns coinciding with the actual configuration of the 3D and four-directional braided composites. A model to predict stiffness was then established based on the stiffness volume average method and flexibility volume average method. The calculated simulation values of the geometric characteristics and elastic constants of braided composites well agree with the measured values, which prove the effectiveness of the model. The present model has presented a foundation for the strength prediction and failure mechanism of the 3D braided composites.