Hypochlorite (ClO-) is widely presented in daily life and human body. An appropriate amount of hypochlorite may help the body regulate the immune system, while excessive hypochlorite can cause a variety of diseases. Herein, a novel fluorescence probe PTZ-BT containing a large conjugate plane was designed and synthesized. It exhibited a large Stokes shift of 158 nm and extreme ME activity and emitted deep-red fluorescence (similar to 642 nm). Moreover, PTZ-BT showed specific turn-off fluorescent-responsive and high-sensitive detection (detection limit equals to 0.64 mu M) toward ClO- in PBS buffer solutions. Job's plot showed that the binding stoichiometry of PTZ-BT with ClO- was 1:1. It was further demonstrated that sulfur atoms in the phenothiazine groups were oxidized to sulfoxide by ClO-, resulting in fluorescence quenching supported by high-resolution mass spectrometry. In addition, PTZ-BT was used for bioimaging in HepG-2 cells with low cell toxicity and good cell permeability. Furthermore, live cells studies also demonstrated PTZ-BT can detect exogenous ClO- inside live cells.