Disclosed is a poly-octavinyl polyhedral oligomeric silsesquioxane-acrylic acid nano composite auxiliary agent for leather. The preparation method for the auxiliary agent comprises: mixing octavinyl polyhedral oligomeric silsesquioxane, an organic solvent, ammonium persulfate, distilled water and a surfactant, dropwise adding acrylic acid, subjecting same to a constant-temperature reaction and cooling to obtain the poly-octavinyl polyhedral oligomeric silsesquioxane-acrylic acid nano composite auxiliary agent for leather. By using the auxiliary agent, the multi-point combination of an acrylic polymer and collagen can be further improved, the filling property to the leather is improved, the leather yield is not affected, and the content of chromium in the waste liquid is reduced.