Free radicals and their induced oxidative damage in living organisms are related to many diseases. Natural substances with antioxidant capacity are effective in scavenging free radicals, which could slow down aging and prevent diseases. However, the existing methods for the evaluation of antioxidant activity mostly required the use of complex instruments and operations. In this work, we proposed a unique method to determine the total antioxidant capacity (TAC) in real samples through a photosensitization-mediated oxidation system. N- and P-doped long-lived phosphorescent carbon dots (NPCDs) were developed, which exhibited the effective intersystem crossing from the singlet to the triplet state under UV light irradiation. Mechanism study confirmed that the energy of excited triplet state in NPCDs generated superoxide radicals and singlet oxygen through type I and type II photoreactions, respectively. On this basis, the quantitative determination of TAC in fresh fruits was achieved using 3,3',5,5'-tetramethylbenzidine (TMB) as a chromogenic bridge in the photosensitization-mediated oxidation system. This demonstration will not only provide a facile way to analyze antioxidant capacity in practical samples but also broaden the applications of phosphorescent carbon dots.