An activatable probe able to detect RONS level underlying the development of rheumatoid arthritis (RA) would be far-reaching for the diagnosis and drug efficacy assessment of RA. Despite more and more evidence suggests that ONOO− is an important signaling molecule participating in the RA disease, only rare fluorescent probes can detect ONOO− in this disease with satisfying performance. To this end, we designed and synthesized a novel activatable AIE fluorescent probe (DPPO-PN) for the detection of ONOO− levels in RA. The probe linearly responds to 0–10 μM ONOO− with a significant far-red fluorescence enhancement at 632 nm in 30 s (F/F0 = 161-fold, LOD = 10 nM) upon the excitation of 490 nm, elucidating excellent sensing abilities for ONOO− in cuvettes. Moreover, the fluctuations of intracellular ONOO− levels caused by adscititious adding or stimulant provoking could be real-time captured and visualized with this probe by confocal imaging. Further, the intravital imaging of ONOO− in LPS-induced and CFA-induced RA mouse models also can be achieved with the aid of the probe, substantiating the burst of ONOO− in the RA process. Therefore, this work not only offers an auxiliary tool for the diagnosis of RA but also would benefit our understanding of the ONOO−‘s roles in RA.