A　fast,　sensitive,　colorimetric　method　for　the　detection　of　pH　based　on　the　differentiate　effect　of　gold　nanoparticles　to　the　configuration　of　DNA　was　developed　in　this　study.　The　UV-Vis　absorption　spectroscopy　of　the　i-motif　DNA-Au　NPs　system　has　been　investigated,　and　the　effect　of　the　concentration　of　salt　and　i-motif　DNA,　reaction　time　and　DNA　sequence　on　the　pH　response　of　the　system　have　been　also　optimized.　Under　the　optimum　conditions,　the　UV-Vis　absorption　spectroscopy　of　the　Au　NPs　is　changed　regularly　with　pH　in　the　range　of　5.　3　similar　to　7.　0,　the　absorbance　at　520　nm　is　increased　gradually　while　at　700　nm　decreased.　Correspondingly,　the　color　of　the　Au　NPs　is　varied　from　violet　to　red.　The　pH　sensor　is　no　need　to　modification,　low　cost,　fast　and　can　be　carried　out　by　naked　eyes.　It　is　promising　to　use　in　monitoring　some　life　process　which　associated　with　pH　variation.