Photocatalytic H2O2 generation system based on polymer catalyst receives increasing attention in recent years; however, the insufficient charge separation efficiency and low oxygen adsorption/activation capacity severely limit their potential application. In this study, a sulfur (C=S) functionalized polymer catalyst is reported through a green water‐mediated and catalyst‐free multi‐component reactions (MCRs) route. The sulfur functional group endows the polymer with a suitable energy band and facilitates the separation of photogenerated electron‐hole pair. The reported polymer achieves a high H2O2 production efficiency (3132 μmol g‐1 h‐1) in pure water without oxygen aeration. To demonstrate their potential in in‐situ wastewater treatment, a panel reactor system (20×20 cm) is constructed for large‐scale production of H2O2, which realizes continuous degradation of emerging pollutants including antibiotics and bisphenol A under natural sunlight irradiation condition. The H2O2 utilization efficiency of the photo‐self‐Fenton system using in‐situ generated H2O2 is found 7.9 times higher than that of the traditional photo‐Fenton system. This study offers new insights in green synthesis and design of functional polymer photocatalyst, and demonstrates the feasibility of panel reactor system for large‐scale continuous H2O2 photocatalytic production and water treatment.