Oxidative stress induced by reactive oxygen species (ROS) plays crucial roles in a wide range of physiological processes and is also implicated in various diseases, including cancer, chronic inflammatory diseases, and neurodegenerative disorders. Among the various ROS, hypochlorous acid (HOCl) plays as a powerful microbicidal agent in the innate immune system. The regulated production of microbicidal HOCl is required for the host to control the invading microbes. However, as a result of the highly reactive and diffusible nature of HOCl, its uncontrolled production may lead to an adverse effect on host physiology. Because of its biological importance, many efforts have been focused on developing selective fluorescent probes to image ROS. However, it is still challenging to design a fluorescent probe with exclusive selectivity toward a particular member of ROS. In the current work, we designed FBS as a new fluorescent HOCl probe which has high selectivity, sensitivity, and short response time in a broad range of pH. Compared with other sensors, the "dual-lock" structure of FBS has an advantage of eliminating interferences from other ROS/RNS. Importantly, we further showed that our HOCl probe could be applied for the in vivo imaging of physiological HOCl production in the mucosa of live animals. This probe provides a promising tool for the study of HOCl production.