Type-I photodynamic therapy (PDT) with less oxygen consumption shows great potential for overcoming the vicious hypoxia typically observed in solid tumors. However, the development of type-I PDT is hindered by insufficient radical generation and the ambiguous design strategy of type-I photosensitizers (PSs). Therefore, developing highly efficient type-I PSs and unveiling their structure-function relationship are still urgent and challenging. Herein, we develop two phenanthro[9,10-]imidazole derivatives (AQPO and AQPI) with aggregation-induced emission (AIE) characteristics and boost their reactive oxygen species (ROS) generation efficiency by reducing singlet-triplet splitting (Δ). Both AQPO and AQPI show ultrasmall Δ values of 0.09 and 0.12 eV, respectively. By incorporating electron-rich anisole, the categories of generated ROS by AIE PSs are changed from type-II (singlet oxygen, O) to type-I (superoxide anion radical, O and hydroxyl radical, •OH). We demonstrate that the assembled AQPO nanoparticles (NPs) achieve a 3.2- and 2.9-fold increase in the O and •OH generation efficiencies, respectively, compared to those of AQPI NPs (without anisole) in water, whereas the O generation efficiency of AQPO NPs is lower (0.4-fold) than that of AQPI NPs. The small Δ and anisole group endow AQPO with an excellent capacity for type-I ROS generation. In vitro and in vivo experiments show that AQPO NPs achieve an excellent hypoxia-overcoming PDT effect by efficiently eliminating tumor cells upon white light irradiation with good biosafety.