Monitoring　mitochondrial　pH　and　differentiating　live　and　dead　cells　are　crucial　for　diagnosing　cell　status.　However,　most　fluorescent　probes　suffer　from　limitations　such　as　high　cytotoxicity,　photobleaching,　unreliability,　and　an　inability　to　differentiate　cell　death　caused　by　different　inducers.　Herein,　a　ratiometric　fluorescent　nanoprobe　was　developed　by　assembling　pH-sensitive　hydroxyl-　and　phenyl-co-modified　carbon　nitride　(HPCN)　with　pH-insensitive　Rhodamine　B　(RB).　HPCN　was　prepared　via　thermal　condensation　of　phenylguanidine　carbonate　using　NaOH　as　the　melt.　The　hydroxyl　group　modification　endowed　HPCN　with　improved　water　solubility　and　pH-sensitive　characteristics,　while　the　phenyl　group　modification　facilitated　mitochondrial　targeting　and　DNA　staining　via　hydrophobic　interactions.　Based　on　the　fluorescence　resonance　energy　transfer　(FRET)　from　HPCN　to　RB,　the　nanoprobe　exhibited　a　linear　response　in　the　relative　fluorescence　intensities　at　500　nm　and　584　nm　over　a　pH　range　of　4.5-8.5.　Benefiting　from　its　low　cytotoxicity,　excellent　reversibility,　and　outstanding　photostability,　the　nanoprobe　was　capable　of　monitoring　mitochondrial　pH　changes　in　live　cells　and　differentiating　live　and　dead　cells,　apoptosis　and　necrosis,　and　necrosis　induced　by　different　agents,　regardless　of　cell　type.　This　work　provides　a　reliable　method　for　diagnosing　cell　status　and　cell　death　induced　by　various　inducers.