Mitochondria are important subcellular organelles involved in many cellular activities. Therefore, it is very important to monitor the concentration of various substances in mitochondria. In this work, we constructed a dual-response mitochondria-targeted fluorescent probe HBTN for the detection of viscosity and HOCl in vitro and in vivo. HBTN not only has a long emission wavelength with an emission peak of 680 nm, but also has a large Stokes shift of 278 nm. The fluorescence intensity of probe HBTN at 680 nm has a good linear relationship with solution viscosity, which can be used for quantitative detection of viscosity. In addition, the probe HBTN enables ratiometric detection of HOCl with a low detection limit (DL = 24.5 nM) and rapid response (<20 min). More importantly, the probe HBTN has been successfully applied to the detection of viscosity and exogenous/endogenous HOCl in the mitochondria of MCF-7 cells. Furthermore, the probe HBTN has been implemented in imaging viscosity and HOCl in zebrafish. HBTN is expected to be a practical tool for monitoring viscosity and HOCl in vivo.