High　hydrogen　sulfide　(H2S)　expression　has　been　shown　to　play　a　crucial　role　in　tumor　growth　and　proliferation　in　colon　cancer.　So　far,　most　probes　used　for　monitoring　H2S　offer　single‐modal　imaging,　which　makes　the　collection　of　accurate　information　for　tumor　diagnosis　difficult.　In　the　present　study,　a　novel　dual‐modal　imaging　probe　in　the　second　near‐infrared　(NIR‐II,　950‐1700)　window　is　proposed　and　constructed　by　coating　a　uniform　shell　of　polydopamine　(PDA)　on　the　surface　of　cuprous　oxide　(Cu2O)‐conjugated　downconversion　nanoparticles　(DCNPs).　Of　note,　DCNP‐Cu2O@PDA　is　in　the　“on”　state,　with　emission　centered　at　1550 nm　(irradiation　using　a　980nm　laser)　in　non‐neoplastic　tissues.　Colon　tumors　with　endogenous　H2S　overexpression　can　induce　the　in　situ　transformation　of　Cu2O　to　copper　sulfide　(Cu2‐XS）.　Owing　to　the　competitive　absorption　between　Cu2‐XS　and　DCNPs　at　a　wavelength　of　980 nm,　the　generated　DCNP‐Cu2‐XS@PDA　exhibits　a　quenched　NIR‐II　fluorescence　(FL)　signal.　Meanwhile,　Cu2‐XS　exhibits　a　strong　absorbance　at　1250 nm,　enhancing　the　NIR‐II　photoacoustic　(PA)　signal　accompanied　by　NIR‐II　FL　attenuation.　The　switching　in　PA　and　FL　signals　at　the　tumor　site　offered　“double　assurances”　for　the　accurate　monitoring　of　H2S.