Peroxynitrite　(ONOO-)　is　a　highly　reactive　nitrogen　species　that　plays　pivotal　roles　in　cell　signal　transduction　and　physiological　or　pathological　progresses.　However,　commonly　used　ONOO-　optical　imaging　probes　are　still　hampered　by　high　background/autofluorescence　(fluorescence　probe),　short　emission　wavelength,　or　poor　selectivity　in　the　case　of　chemiluminescence.　Herein,　we　report　a　facile　method　to　prepare　an　activatable　chemiluminescence　probe　(PPA-PEG)　with　good　biocompatibility　and　functionality　for　in　vivo　autofluorescence-free　imaging　of　ONOO-.　The　PPA-PEG　consists　of　pyropheophorbide-a　(PPA),　a　typical　deep　red　photosensitizer　that　acts　as　both　the　recognition　and　signaling　element,　and　4-arm　poly(ethylene　glycol)　(4-arm　PEG),　which　improves　the　biosafety　and　water　solubility　of　probe.　These　components　can　self-assemble　into　nanoparticles　(namely　PPA-PEG　nanoprobe)　in　aqueous　solution.　The　PPA-PEG　nanoprobe　showed　an　ultra-low　chemiluminescence　signal　before　interacting　with　ONOO-,　but　exhibited　good　selectivity,　high　sensitivity　and　a　fast　response　toward　ONOO-.　The　PPA-PEG　was　successfully　applied　to　image　cellular　ONOO-　changes,　as　well　as　the　endogenous　ONOO-　changes　in　inflammation　models　and　subcutaneous　or　orthotopic　hepatocellular　carcinoma　(HCC)　tumors　models　in　living　mice.　In　vitro　and　in　vivo　studies　verified　the　good　detection　and　imaging　capabilities　of　PPA-PEG　for　peroxynitrite,　demonstrating　suitable　tissue　penetration　and　a　high　signal-to-background　ratio　(SBR).　Thus,　our　nanoprobe　can　serve　as　a　valuable　activatable　chemiluminescence　imaging　tool　for　studying　important　peroxynitrite-related　chemical　and　biological　applications.