The peroxymonosulfate-based advanced oxidation process (PMS-AOP) is widely studied as a powerful technology for organic pollutants degradation in wastewater. In this paper, an inventive one-step method utilizing potassium persulfate (KPS) as an oxidizing agent was put forward to prepare metal-free carboxyl modified graphite phase carbon nitride (g-C3N4(KPS)), which was applied to significantly activate PMS for the degradation of organic compound carbamazepine (CBZ) under solar radiation. Without strong acids and organic solvents, the process was considered green, low-cost, and facile. The characterizations showed that g-C3N4(KPS) was successfully doped with carboxyl groups on g-C3N4. Besides, •OH, 1O2, •O2− and h+ were generated in the photocatalytic system of g-C3N4(KPS)/PMS. As a result, a possible CBZ photocatalytic degradation pathway was proposed. This work not only discovered a novel method of activating PMS but also offered valuable insights for the preparation of efficient metal-free catalysts for wastewater environmental governance.