Singlet　oxygen　(O-1(2))　has　a　potent　anticancer　effect,　but　photosensitized　generation　of　O-1(2)　is　inhibited　by　tumor　hypoxia　and　limited　light　penetration　depth.　Despite　the　potential　of　chemodynamic　therapy　(CDT)　to　circumvent　these　issues　by　exploration　of　O-1(2)-producing　catalysts,　engineering　efficient　CDT　agents　is　still　a　formidable　challenge　since　most　catalysts　require　specific　pH　to　function　and　become　inactivated　upon　chelation　by　glutathione　(GSH).　Herein,　we　present　a　catalytic　microenvironment-tailored　nanoreactor　(CMTN),　constructed　by　encapsulating　MoO42-　catalyst　and　alkaline　sodium　carbonate　within　liposomes,　which　offers　a　favorable　pH　condition　for　MoO42--catalyzed　generation　of　O-1(2)　from　H2O2　and　protects　MoO42-　from　GSH　chelation　owing　to　the　impermeability　of　liposomal　lipid　membrane　to　ions　and　GSH.　H2O2　and　O-1(2)　can　freely　cross　the　liposomal　membrane,　allowing　CMTN　with　a　built-in　NIR-II　ratiometric　fluorescent　O-1(2)　sensor　to　achieve　monitored　tumor　CDT.