Carbon　nanomaterials,　such　as　graphene　and　its　derivatives　can　act　as　a　significant　building　nanomaterial　for　ameliorating　the　various　properties　of　cement-based　composites,　but　their　poor　water　solubility　lead　to　unevenly　dispersed　in　the　cement　pore　solution,　and　limits　their　effective　application　in　cement-based　materials.　Moreover,　the　high　price　of　these　nanomaterials　restricts　the　practical　use　in　industry.　Herein,　in　this　study,　the　carbon　dots　(CDs),　which　was　prepared　by　the　improved　microwave　pyrolysis　method,　is　demonstrated　to　own　the　superior　performance　in　the　enhancement　of　the　cement-based　composites　mechanical　properties　for　the　first　time.　The　Xray　powder　diffraction　(XRD),　High-resolution　transmission　electron　microscopy　(HR-TEM),　X-ray　photoelectron　spectroscopy　(XPS)　and　related　structural　characterizations　are　performed　to　prove　the　successful　synthesis　of　CDs,　and　the　compressive　strength　of　cement-based　composites　increased　by　16.8%　after　curing　28　days　at　the　0.08　wt%　CDs　dosage　in　comparison　to　the　blank　samples.　Through　thermogravimetric　analysis　(TGA),　scanning　electron　microscopy　(SEM)　and　mercury　intrusion　porosimeter　(MIP)　and　XRD,　it　is　proved　that:　i)　the　small　nanometer　size　and　good　water　solubility　of　carbon　dots　promote　them　easily　filling　into　the　pores　of　cement　as　a　result　of　increasing　its　compactness;　ii)　the　carbon　dots　act　as　much　more　nucleation　sites　to　make　the　hydration　products　superimposed　in　a　more　dense　room;　iii)　the　surface　of　carbon　dots　possess　many　hydrophilic　groups,　including　carboxyl　and　amide,　these　groups　produce　a　strong　adsorption　on　the　cement　particles　and　then　affect　the　formation　of　hydration　products.　These　joint　effects　effectively　enhance　the　compressive　mechanical　strength　for　cement-based　composites.　Besides,　the　simple　preparation　of　CDs　with　a　low　price　is　suitable　for　the　industrial　application.　The　CDs　as　the　superior　alternative　nanomaterial　for　heightening　the　properties　of　cement-based　composites　show　a　great　potential,　and　is　significant　for　the　exploitation　of　building　nanomaterial　with　outstanding　performances　for　cement.