Abstract: For the 3D five-direction tubular preform produced by four-step braiding, the yarn geometric topologies in the interior and the surface regions were studied, and a unit-cell structure model was presented on the basis of tubular braiding process and principles. Under these conditions, the yarn trajectory projection was simulated as the Archimedean screw in the interior region or the circular arc in the surface region and characterized with a corresponding mathematical formula. On the other hand, the fiber volume fraction of different zones in the fabric's cross-section was computed in terms of the unit-cell's geometric feature. Then the preform's architecture and the fiber volume fraction in a cross-section were simulated by building the yarn's data structure and using the computer visualization techniques, which may have a great meaning to characterize the structure of the 3D five-direction tubular preform and optimize the braiding parameters.