It is still a huge challenge to design a high‐efficiency ultrathin mesoporous nonmetallic photocatalyst by facile eco‐friendly approach for simultaneously enhanced photocatalytic performance for degradation organic pollutant and H2 generation from water splitting. Herein, ultrathin mesoporous carbon nitride nanosheets are prepared via a one‐step thermal polycondensation approach using urea and ammonium carbonate ((NH4)2CO3) as precursor, where the small molecules generated in the thermal treatment process are environmentally friendly. It exhibits high photocatalytic degradation bisphenol A performance and H2 evolution activity compared with the bulk graphitic carbon nitride (g‐C3N4) nanosheet. The improvement of photocatalytic performance results from the ultrathin mesoporous structure with enlarged specific surface area and the abundant mesoporous channels, which facilitates the light absorption and promotes the separation efficiency of photogenerated electron–hole pairs. This work demonstrates a facile eco‐friendly approach to modulate g‐C3N4 morphology for improving its photocatalytic performance.