Aiming at a more mechanically optimized braided preform and its manufacturing, thick square braids with hollows were investigated and the method for determining bundle parts of the 3D braids were developed. Formulas giving the optimum number of bundles of the square braids were derived and a design method to arrange horn gears and to determine the number of horn docks, necessary for the construction of the 3D rotary braider, were also presented. These lead to the manufacture of 3D braids with arbitrary braiding structures. On the basis of the proposed theory, a rotary braider capable of manufacturing any 3D braids with a wide variety of cross sections was designed by unitizing the spindle-exchange mechanism. The realized braider worked successfully and manufactured thick 3D braids with solid and hollow cross sections.