A preparing method for coating polymethylmethacrylate (PMMA) particles with silicon dioxide is disclosed and includes the following steps of: preparing a silicon dioxide solution by mixing a silicon dioxide powder and a solvent; adding a dispersant-and-interface-modifier agent into the silicon dioxide solution; performing a wet grinding to the silicon dioxide solution with the dispersant-and-interface-modifier agent so as to obtain a plurality of nano-sized silicon dioxide particles with negative charge; performing an interface modification to a plurality of PMMA particles to be charged with positive charge; adding the PMMA particles into the silicon dioxide solution; making the PMMA particles adsorb the nano-sized silicon dioxide particles; and performing a solid-liquid separation process to the silicon dioxide solution so as to obtain the chemical composite particles.