To attempt to bridge the relationships between the braiding procedure and finite element model for 3D braided structural preform plates, this paper develops a software that automatically generates the mesoscale finite element model consisted in 8-node hexahedron elements of 3D braided preform with requiring only braiding angle, dimensions and cross-section of yarns as input parameters. Taking three different representative volume elements into account, the geometric description of 3D braided preform is established with the assumptions that the braided yarns with the circle cross-section keep straight in the interior, bend and change linearly to other directions at the surface and corner regions. After that, all yarns are divided into segments and classified into eight groups according to centreline of segment orientations, which leading to easily assign material orientations and properties for transversely isotropic constitutive of yarns. Considering the different yarns jamming effect in different regions, the model involves the various diameters for yarns in the three regions, which also can ensure no interpenetrations among yarns. What’s more, a mesoscale finite element model involving matrix pockets is developed for 4-step three-dimensional rectangular braided composites subjected to a steel ball with an initial velocity of 200 m/s in LS-DYNA to validate its practicality.