The　effect　of　diammonium　hydrogen　phosphate　(DAP)　on　the　setting　time,　mechanical　properties　and　micro-nanostructure　of　cement　paste　is　investigated.　The　results　show　that　with　an　increase　in　DAP　content,　the　peak　value　of　hydration　heat　of　cement　is　delayed,　but　the　3d　cumulative　hydration　heat　release　of　the　cement　is　basically　the　same.　The　3d　compressive　strength　of　cement　paste　is　not　significantly　different,　and　the　28d　compressive　strength　increases　first　and　then　decreases.　When　the　DAP　content　is　0.5　wt%,　the　compressive　strength　of　cement　paste　is　most　significantly　improved,　increasing　by　38.5%.　Meanwhile,　the　initial　and　final　setting　times　of　cement　paste　are　extended　by　74.0%　and　73.2%,　respectively.　When　the　DAP　content　is　1.0　wt%,　the　cement　paste　has　the　longest　setting　time,　with　the　initial　and　final　setting　times　extended　by　101.8%　and　103.5%,　respectively.　However,　the　compressive　strength　of　the　cement　paste　can　still　be　improved　by　10.5%.　It　is　mainly　due　to　the　change　of　pH　value　and　calcium　ion　concentration　of　cement　pore　solution　with　different　DAP　content,　which　changes　the　micro-nanoscale　morphology　of　C-S-H　gel,　Hamaker　constant　between　globules,　microscopic　Young＇s　modulus,　and　pore　structure.