A ratiometric fluorescent probe NR for HOCl based on a through-bond energy transfer (TBET) strategy was developed, which responded to HOCl with short response time, high selectivity, high photostability and lysosomal-targeting ability. NR was applied to monitor artificially loaded and endogenous HOCl in RAW264.7 cells. Display omitted •A ratiometric HOCl fluorescent probe was designed based on the naphthalimide-rhodamine TBET platform.•The probe showed high selectivity and excellent sensitivity toward HOCl.•The probe was suitable for imaging exogenous and endogenous HOCl in the living cells.•The probe could selectively stain lysosomes. A ratiometric fluorescent probe (NR) for HOCl based on a new naphthalimide-rhodamine platform involving through-bond energy transfer (TBET) process is presented. Rhodamine B thiohydrazide and naphthalimide derivative were linked to form a structure of monothio-bishydrazide. Free NR only emitted the fluorescence of naphthalimide. The treatment of NR with HOCl converted the monothio-bishydrazide into 1,2,4-oxadiazole. As a result, naphthalimide donor and rhodamine acceptor were connected with electronically conjugated bonds, and the spiro ring of rhodamine was opened. Hence, a typical TBET process took place, resulting in the increase of rhodamine emission at 582nm as well as the decrease of naphthalimide emission at 532nm. I582/I532 ratios showed desirable HOCl-dependent performance and responded linearly to minor changes of HOCl concentration (0–5μM). The detection limit was determined to be 14.5nM. NR exhibited high selectivity, rapid response and low cytotoxicity. Moreover, NR has been employed for the imaging of artificially loaded HOCl and endogenously produced HOCl with satisfying results. Therefore, NR may act as a practical molecular tool for monitoring HOCl fluctuations in biological systems.