In　this　work,　uniformly-dispersed　platinum　nanoparticles　(PtNPs)　were　synthesized　by　a　simple　chemical　reduction　method,　in　which　citric　acid　and　sodium　borohydride　acted　as　a　stabilizer　and　reducer,　respectively.　An　ultrasensitive　colorimetric　sensor　for　the　facile　and　rapid　detection　of　Ag+　ions　was　constructed　based　on　the　peroxidase　mimetic　activities　of　the　obtained　PtNPs,　which　can　catalyze　the　oxidation　of　3,3＇,5,5＇-tetramethylbenzidine　(TMB)　by　H2O2　to　produce　colored　products.　The　introduced　Ag+　would　be　reduced　to　Ag-0　by　the　capped　citric　acid,　and　the　deposition　of　Ag-0　on　the　PtNPs　surface,　can　effectively　inhibit　the　peroxidase-mimetic　activity　of　PtNPs.　Through　measuring　the　maximum　absorption　signal　of　oxidized　TMB　at　652　nm,　ultra-low　detection　limits　(7.8　pM)　of　Ag+　can　be　reached.　In　addition　to　such　high　sensitivity,　the　colorimetric　assay　also　displays　excellent　selectivity　for　other　ions　of　interest　and　shows　great　potential　for　the　detection　of　Ag+　in　real　water　samples.