To　explore　the　effects　of　sea-sand　as　fine　aggregate　on　the　hydration　and　microstructure　evolution　mechanism　of　ultra-high　performance　concrete　(UHPC),　UHPC　was　prepared　with　simulated　sea-sand　with　different　chloride　ion　contents.　The　evolution　of　micro-environment　in　UHPC　matrix　of　sea-sand　was　studied　by　isothermal　conduction　calorimetry,　X-ray　diffraction,　determination　test　of　chloride　ion　content　and　pH　content　in　matrix,　scanning　electron　microscopy,　energy　spectrum　analysis,　and　nitrogen　adsorption　and　desorption.　The　results　show　that　the　hydration　process　of　UHPC　increases　with　the　increase　of　the　chloride　ion　content.　The　Friedel　salt　is　not　formed　in　the　matrix　at　3　d,　but　at　28　d.　The　chloride　binding　behavior　of　sea-sand　UHPC　mainly　occurs　before　7　d.　The　capacity　of　the　UHPC　to　bind　chloride　ions　decreases　with　the　increase　of　the　chloride　ion　content.　The　pH　value　of　the　matrix　decreases　with　the　increase　of　the　chloride　ion　content　after　hydration　for　14　d,　and　is　gradually　stable　at　28　d.　The　incorporation　of　sea-sand　can　refine　the　pore　size　of　2　to　10　nm　in　the　early　stage,　but　the　total　porosity　of　UHPC　is　no　different　from　that　of　the　chlorine-free　UHPC　in　the　later　stage.　The　results　provide　a　theoretical　basis　for　the　excellent　mechanical　properties　and　the　reinforcement　protection　property　of　sea-sand　UHPC.　©　2022,　Editorial　Department　of　Journal　of　Southeast　University.　All　right　reserved.