Photodynamic therapy (PDT) as a burgeoning noninvasive therapeutic method has been utilized in preclinical treatment thanks to its easy accessibility, low toxicity, high spatiotemporal precision, and less incidence to evolve drug resistance. Photosensitizer (PS) is the primary element of PDT, which can generate two kinds of reactive oxygen species (ROS) upon photoexcitation to ablate cancer cells. One is type I ROS depending less on oxygen, such as hydroxyl radical (OH • ), superoxide radical (O2 •-) and hydrogen peroxide (H2O2), which shows great potential for antitumor treatment under hypoxia. The other is type II ROS including singlet oxygen ( 1 O2). Besides the efficient therapy capability, fluorescence photosensitizers can visualize the tumor locations and image organelles in cancer cells as well. Thus, imageguided PDT is a versatile theranostic technique. While, the "always on" mode of photosensitizers contributes to a low target-to-background ratio on account of a high background interference. Therefore, development of "off-on" photosensitizer system with high sensitivity to tumor microenvironment change is extremely desirable to detect cancer in the initial phase.