Cu-based　perovskites　have　excellent　optical　properties,　environmental　friendliness,　and　broad　optical　applications.　However,　their　development　is　hindered　by　complex　preparation　methods　and　poor　hydrochromic　stability.　In　this　work,　a　solvent-evaporation-based　crystallization　strategy　combined　with　Sn　doping　for　the　preparation　of　Cs3Cu2I5　microcrystals　(MCs)　is　reported.　This　strategy　enhances　the　photoluminescence　quantum　yield　(PLQY)　and　hydrochromic　reliability.　In　addition,　first-principles　calculations　reveal　the　effect　of　Sn　doping　on　the　lattice　structure　and　exciton–phonon　coupling　of　the　Cs3Cu2I5　MCs.　The　results　show　that　Sn　doping　reduces　the　lattice　spacing　as　a　result　of　the　substitution　radius　difference　between　Cu+　and　Sn2+　ions,　which　causes　stronger　exciton–phonon　coupling　and　enhances　the　PLQY.　Moreover,　the　Sn-doped　Cs3Cu2I5　MCs　show　excellent　reliability　on　heating　and　under　hydrochromic　cycles　and　long-term　luminescence　conditions.　Based　on　their　hydrochromic　and　temperature-responsive　properties,　the　Sn-doped　Cs3Cu2I5　MCs　are　applied　as　encryptable　printing　materials　for　information　encryption　and　decryption　and　as　a　fluorescence-based　temperature　sensor,　combined　with　Machine-Learning　to　guide　the　manufacturing　of　fluorescent　thermometers.　The　findings　provide　insights　into　the　commercial　value　of　copper-based　perovskites　and　demonstrate　their　potential　anti-counterfeiting,　fluorescence　temperature　sensing　applications.　©　2023　Wiley-VCH　GmbH.