An　enzyme-catalyzed　fluorescence　＂switch＂　type　sensor　was　constructed　for　the　determination　of　alkaline　phosphatase　(ALP)　activity　by　combining　the　fluorescence　quenching　effect　of　Ag(+)on　ultrathin　g-C(3)N(4)nanosheets　(CNNSs)　with　the　simple　redox　reaction　of　AA　and　Ag+.　Briefly,　Ag(+)exhibits　a　significant　quenching　effect　on　the　fluorescence　of　CNNSs.　Thus　the　fluorescence　signal　of　the　CNNS-Ag(+)system　is　extremely　weak　even　in　the　presence　ofl-ascorbic　acid-2-phosphate　(AAP)　(＂off＂　state).　When　ALP　coexists　in　the　system,　the　enzyme　can　specifically　catalyze　the　hydrolysis　of　AAP　to　form　ascorbic　acid　(AA),　which　reduces　Ag(+)to　Ag-0.　In　this　case,　the　fluorescence　signal　of　the　system　is　recovered　(＂on＂　state).　Based　on　this　principle,　a　signal-enhanced　CNNS　fluorescence　sensor　was　developed　to　determine　the　activity　of　alkaline　phosphatase.　The　experimental　results　show　that　the　detection　range　of　alkaline　phosphatase　is　0.5-20　U　L-1,　and　the　detection　limit　is　0.05　U　L-1(S/N　=　3).　Meanwhile,　this　method　was　used　to　assay　ALP　in　serum　samples.