Tubular braiding fabric is widely used in developing tissue-engineered scaffolds, and is especially suitable for connective tissues like ligaments and tendons. The pore size and porosity of braiding structure scaffolds not only highly affect cell adhesion and proliferation, but also influence the mechanical behavior of those scaffolds. It is important to develop braiding scaffolds with controllable pore size and distribution. The purpose of this work is to add insight to the mechanics of this passive pore structure control system. Thus, some constitutive equations were established to reveal the relationship between braiding technical parameters (including the number of spindles, braiding structure, cylindrical mandrel radius, and yarn diameter) and the pore size, the porosity of tubular braiding fabric by the mathematical modeling method. Through this model, pore size and the porosity of the tubular braiding scaffold can be precisely controlled by quantitatively adjusting braiding technical parameters. Furthermore, the reliability and accuracy of this model were verified by the experimental data.