Ascorbic　acid　(AA)　and　alkaline　phosphatase　(ALP)　in　human　serum　are　indispensable　substances　in　the　process　of　biological　metabolism,　and　their　abnormal　contents　are　strongly　associated　with　a　range　of　diseases.　Therefore,　it　is　meaningful　to　develop　a　simple,　accurate　and　sensitive　assay　for　the　detection　of　AA　and　ALP.　Up-conversion　nanoparticles　(UCNPs)　have　some　unique　characteristics,　especially,　the　potential　for　background-free　excitation　by　the　near-infrared　region　(NIR)　significantly　improves　the　limits　of　detection　(LODs).　Moreover,　sensors　based　on　the　inner　filter　effect　(IFE)　do　not　require　covalent　linkage　between　receptors　and　fluorophores　or　surface　modification,　thus　the　probe　fabrication　is　simple　and　flexible　UCNP-based.　Therefore,　a　simple,　selective,　sensitive,　and　flexible　sensing　system　for　the　detection　of　AA　and　ALP　is　proposed　by　combining　the　IFE　and　the　cyclic　signal　amplification　strategy.　Under　ideal　circumstances,　the　LODs　of　ALP　through　the　colorimetric　and　fluorescence　method　were　0.192　U/L　and　0.013　U/L.　The　established　sensing　platform　was　applied　to　assessing　ALP　activity　in　human　serum,　and　the　detection　results　are　close　to　those　of　commercially　available　kits,　indicating　its　immense　potential　as　an　effective　and　convenient　tool　for　monitoring　ALP　activity　in　clinical　settings.