The invention relates to a poly silsesquioxane-acrylic nanocomposite auxiliary for leather process and its preparation method. By mixing octavinyl silsesquioxane, organic solvent, ammonium persulfate, distilled water and surfactant in one system, and adding acrylic acid dropwise into the mixing system, cooling to adjust pH to obtain poly silsesquioxane-acrylic nanocomposite auxiliary for leather process. The auxiliary obtained in this invention is easily to penetrate into gap of the collagen fibers, and forms nano and micro-scale combination with the leather collagen, which can further improve the multi-point combination of the acrylic polymer with the collagen. Thus, the filling property of the leather that treated by the auxiliary is improved without affect the yield of leather, and the chromium content in the waste water is significantly reduced.