Cancer, as a malignant tumor, seriously endangers human health. The study of cancer diagnosis and therapy has great practical significance. The development of theranostic agents has become a very important research topic. Nevertheless, some existing agents still have imperfections, such as complex structures and difficult syntheses. Therefore, it is urgent for researchers to develop simple novel theranostic agents. In this study, the precipitated fluorophore HAPQ was used as a simple drug molecule for the first time and combined with NBD-Cl to construct a simple and efficient theranostic probe (HAPQ-NBD). The theranostic probe can distinguish between tumor cells and normal cells based on the higher levels of biothiol in tumor cells. In addition, the probe can use biothiol as a control switch to release higher levels of precipitated fluorophore HAPQ in tumor cells, leading to selective high toxicity to tumor cells, thus achieving the goal of selectively killing tumor cells. The construction of probe HAPQ-NBD provides a practical tool for the diagnosis and therapy of cancer. It is expected that the development and utilization of precipitated fluorophore will provide a new method and strategy for cancer diagnosis and therapy.