Recently, perovskite is becoming a promising alternative as peroxymonosulfate (PMS) activator for the remediation of organic pollutants in water. But the factor determining PMS activation efficiency of perovskite and the evolution of reactive oxygen species (ROS) remain equivocal and elusive. In this study, we proposed an oxygen defect dependent PMS activation mechanism over perovskite with the singlet oxygen (O-1(2)) as the dominant ROS. Among the tested four perovskites, ofloxacin (OFX) degradation efficiency increased with the following order: LaFeO3 < LaZnO3 < LaMnO3 < LaNiO3, which agreed well with their oxygen defect amounts based on X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analysis. The results clearly demonstrated a good relationship among oxygen defects in LaBO3, OFX degradation efficiency and O-1(2) concentration. Moreover, O-1(2) evolution mechanism over perovskite by decreasing the activation energy of PMS self-decomposition was proposed. The O-1(2) mediated OFX degradation pathway was further studied by HPLC-MS technique and three-dimensional excitation-emission matrix fluorescence spectroscopy (3D EEMs). This work provides a new insight into PMS activation by perovskites and favors its application in actual water treatment.