We present a newly designed colorimetric sensor sensitive to pH value based on a gold nanocomposite composed of gold nanoparticles and pH-sensitive polymer{dodecylthioether end functionalized poly[2-(diethylamino)ethyl methacrylate],poly(DEAEMA)-DDT}.We have shown that this design can produce stable GNP precipitate under weakly basic condition(pH=7.5) and this precipitate can be dispersed in acidic solution(pH=4.0),due to the 2-(diethylamino) ethyl methacrylate protonated by H+.The precipitation-dispersion process is reversible and accompanied by distinct color change that are clearly visible to the naked eye.This development may have application in the controlled fabrication of reversible pH-sensitive nanostructures and/or devices.