The photocatalytic performances of graphitic carbon nitride (GCN) are dramatically limited by rapid charge carriers’ combination and the low utilization of photo-excited electrons and holes thereafter. To address this, we employed a wet impregnation approach to anchor single platinum atoms onto holy carbon nitride nanosheets (PtSA-HCNNS). The X-ray absorption fine structure and density functional theory calculations illustrated that PtSA-HCNNS was equipped with Pt–N4 coordination, which was favorable for oxygen absorption and the generation of reactive oxidation species. As a result, PtSA-HCNNS exhibited significantly enhanced photocatalytic activity for evolving reactive species and degrading herbicide (halosulfuron-methyl) than pristine GCN and Pt cluster-decorated GCN. In addition, PtSA-HCNNS was capable of simultaneously removing five commonly used sulfonylurea herbicides and reducing their phytotoxicity toward soybean seedlings. This work offers a facile approach to developing high-performance porous carbon nitride with single Pt doping for sustainable photocatalytic purification.