The　grouting　quality　of　post-tensioned　prestressed　ducts　is　related　to　the　safety　of　bridges.　An　inspection　system　based　on　coplanar　capacitive　sensor　for　grouting　quality　was　presented,　which　can　detect　the　stratification　of　grouting　in　the　ducts.　With　the　derived　semi-analytical　equation　for　a　double-plate　capacitive　proximity　sensor,　the　optimal　parameters　of　coplanar　capacitive　sensor　were　determined.　Based　on　the　principle　of　capacitive　proximity　sensor,　data　acquisition　unit　and　data　conversion　unit　were　designed.　To　promote　the　system,　system　control　unit　and　data　display　unit　were　designed.　According　to　the　approximation　theory,　the　system　verification　test　was　designed　by　measuring　capacitance　of　different　thicknesses　of　liquid　on　the　glass　surface　and　comparing　the　measurement　results　with　the　COMSOL　simulation　results,　error　analysis　and　data　processing　were　done.　Finally,　a　system　error　evaluation　was　completed.　Experiments　show　that　the　average　relative　error　of　the　system　is　9.5%,　the　resolution　is　0.1　mm　and　the　system　can　effectively　distinguish　stratification　of　grouting　in　the　ducts.　Additionally,　the　inspection　system　size　(except　the　sensor)　is　100　mm×80　mm×30　mm　and　is　equipped　with　a　portable　power　that　is　convenient　for　online　inspection.　The　system　can　be　used　in　inspection　of　grouting　quality　of　post-tensioned　prestressed　ducts　and　the　test　result　is　reliable　and　accurate,　which　can　satisfy　the　requirements　of　engineering　application.　©　2016,　Beijing　University　of　Technology.　All　right　reserved.