The　use　of　seawater　and　sea　sand　for　the　preparation　of　alkali-activated　concretes　plays　an　important　role　in　reducing　carbon　emissions　and　alleviating　resource　scarcity.　However,　both　alkali-activated　concretes　and　products　made　from　seawater　and　sea　sand　tend　to　exhibit　significant　drying　shrinkage.　Therefore,　this　study　investigates　the　effect　of　the　addition　of　seawater　and　sea　sand　on　the　basic　properties　and　drying　shrinkage　properties　of　alkali-activated　concretes,　and　regulates　the　properties　of　seawater　and　sea　sand　alkali-activated　slag　(SSAS)　concretes　with　the　key　parameter　of　the　alkali　equivalent.　Finally,　the　mechanism　of　SSAS　drying　shrinkage　improvement　was　investigated　using　XRD,　TG,　MIP,　and　SEM.　The　results　show　that　the　addition　of　seawater　and　untreated　sea　sand　accelerates　the　hydration　reaction　of　alkali-excited　cement　and　can　significantly　reduce　its　drying　shrinkage.　A　moderate　increase　in　alkali　content　can　improve　the　compressive　strength　and　reduce　the　drying　shrinkage.　However,　an　excessively　high　alkali　content　reduces　the　flexural　strength.　Finally,　we　propose　the　existence　of　a　quantitative　relationship　between　drying　shrinkage,　mass　loss,　and　the　evaporable　water　content,　which　better　explains　the　mechanism　of　shrinkage　variation.　These　effects　are　mainly　attributed　to　changes　in　microstructure　and　phase　composition.　This　study　provides　theoretical　support　for　engineering　applications　of　seawater–sea　sand　alkali-activated　materials.　©　2025　by　the　authors.