A one-step salt melt assisted thermal polymerization for the synthesis of nitrogen-rich water soluble graphitic carbon nitride (WS-GCN) is presented. The as-prepared WS-GCN shows strong tunable excitation-emission pathways that have multi-colored photoluminescence from blue to green with extremely high absolute quantum yields (AQY) of 22%, which is almost 6 fold of pristine graphitic carbon nitride (GCN) nanosheets. It also shows significant red-shift of response edge benefiting from the narrowed band energy, due to the feature of N-defected surface and N-rich inner of WS-GCN. After being decorated onto TiO2, the WS-GCN endows a great enhancement of NO-photodegradation under visible light irradiation, 5.5-fold increase to comparison with what the pristine GCN does. Further, the starting edge of photocatalysis is enlarged to about 650 sun for TiO2/WS-GCN from about 450 nm for TiO2/GCN. This is ascribed to both enhanced light-response range and efficiently restraining the radiative transition probability of WS-GCN owing to the migration of photoinduced electrons from the CB of WS-GCN to TiO2. This multi-functional material may provide a way for the development of bio-imaging, file secrecy and broad absorption photocatalysts.