A　good　water-soluble　and　stable　nanocomposite　has　been　facilely　prepared　by　the　encapsulation　of　CsPbBr3　QDs　via　cetyltrimethylammonium　bromide　and　mineral　oil　through　sonication,　namely　CsPbBr3@CMO　nanocomposite.　Such　method　is　very　quick　and　simple　without　complicated　instruments　and　strict　conditions.　The　results　reveal　that　the　synthesized　CsPbBr3@CMO　nanocomposite　is　spherical　with　uniform　size　and　shows　remarkably　good　solubility　and　stability　in　water.　Specifically,　the　fluorescent　intensity　of　CsPbBr3@CMO　nanocomposite　in　water　is　decreased　by　0.76%　after　3　h;　this　result　is　comparable　with　those　in　earlier　studies,　and　the　good　stability　in　water　might　be　owned　to　the　hydrophobic　core　of　the　CsPbBr3@CMO　nanocomposite.　The　prepared　CsPbBr3@CMO　nanocomposite　has　been　applied　as　a　sensitive　fluorescent　probe　for　monitoring　hydrogen　sulfide　(H2S),　and　the　fluorescence　intensity　(similar　to　524　nm)　has　a　linear　relationship　with　the　concentration　of　H2S　in　the　range　0.15-105　mu　M　with　a　detection　limit　of　53　nM,　demonstrating　application　for　monitoring　H2S　in　rat　brain　coupled　with　microdialysis　apparatus　with　satisfied　results.　The　present　study　not　only　provides　a　simple　but　sensitive　approach　for　the　detection　of　H2S　in　living　body,　but　also　paves　the　way　for　expanding　the　application　of　CsPbBr3　QDs　to　aqueous　medium.