The development of smart photosensitizers with tumor microenvironment-activable fluorescence turn-on and singlet oxygen generation plays an important role in tumor bioimaging and photodynamic therapy. Herein, a pH-activable heavy-atom-free photosensitizer (C60-RB) has been successfully synthesized through introducing a fullerene unit onto rhodamine B hydrazide. Under acidic conditions, C60-RB, having a spirolactam structure, can be activated to its ring-opened structure C60-RB-H and thus, visible-light absorbance enhancement, fluorescence turn-on and triplet excited state generation can be accomplished. In the presence of hydrion, the fullerene unit in C60-RB C60-RB-H acting as an intramolecular spin converter can cause good intersystem crossing, and the energy gap between S1 and T1 (ΔEST) is lowered to 0.017 eV. Through encapsulation with amphiphilic DSPE-mPEG2000, water-soluble nanoparticles (NPs) of C60-RB are obtained. In vitro experiments indicate that C60-RB NPs are capable of universal cellular uptake and lysosomal activation (pH 4.5-5.0), and they also exhibit excellent photodynamic therapeutic effect. The fluorescence turn-on and efficient singlet oxygen generation enabled by pH-activated C60-RB NPs testify their great potential applications for cancer diagnosis and treatment.