Abstract: Based on the existing experiment observation and the movement t races of yarn carriers on the braidingmachine bed , a microst ructure model of 4-step three-dimensional rectangular braided composites is established bythe analytical approach. Especially , the spatial configuration of interior yarns was analyzed in detail , and the distortion of the cross-section of interior yarns caused by mutual contact and jamming was considered in the model. As thejamming conditions in different regions were not uniform , the yarn packing factor of interior yarns was differentfrom that of surface yarns and corner yarns in the model. Then a new unit-cell partition scheme that can overcomethe shortcomings of the existing 45°partition schemes was proposed to make the directions of all the unit-cells parallel with the edges of the cross-section of the composites. Then the microst ructure unit-cell models for the three regions of the composites , the interior , surface and corner were established , which were expediently utilized to establish the prediction models of mechanical properties. By using the unit cell models , the st ructural geometry of thecomposites was analyzed , and the mathematical relation between technics parameters and microst ructure of the unitcells was obtained. The calculated values of the geometric characteristics of braided composites specimens by themodel well agree with the measured values. The numerical results indicate the effectiveness of the model , and thepresent model has presented a foundation for the mechanical performance prediction of the composites.