The persulfate-based advanced oxidation process demonstrates significant advantages in the removal of refractory organic pollutants. However, traditional activation methods are associated with high energy consumption and the potential generation of secondary pollution, which restricts the widespread application of this technology. Based on the photosensitive properties of dissolved organic matter (DOM) in secondary effluent, a practical self-activation peroxymonosulfate (PMS) reaction system using solar radiation was established. Systematic investigations were conducted on the effects of different persulfate types, initial pH conditions, and common inorganic anions on PMS decomposition efficiency, accompanied by comprehensive characterization of physicochemical properties and structural transformations in DOM before and after the reaction. The results indicated that the all fractions of secondary effluent could effectively activate PMS, with the hydrophilic (HI) fraction exhibiting the most efficient activation, achieving a PMS decomposition rate of up to 26.7% under neutral conditions. During the decomposition process, reactive oxidative species generated from PMS interacted with DOM, leading to the gradual degradation of its aromatic structures and a reduction in molecular weight. The intensity of fluorescent substances, such as humic-like compounds, significantly decreased, while the proportions of lignin, lipids, tannins, and other compounds had notable changes. These observations indicated that the DOM in secondary effluent achieved self-degradation through effective activation of PMS. Humic-like compounds and lignin in secondary effluent DOM served as the primary electron donors. Upon excitation by sunlight, they transferred photogenerated electrons to PMS, thereby activating it and triggering the self-degradation of DOM. The self-catalytic system of using the DOM in actual secondary effluent in this study without the need for external catalysts, avoiding high energy consumption and secondary pollution.