The photocatalytic water splitting for hydrogen fuel by utilizing solar energy is considered a useful route to settle the energy crisis, but a bottleneck is occurred in developing a highly efficient photocatalysts. Herein, a facile method is applied to fabricate the holey carbon nitride (HCN) nanosheets with high crystallization network and extended optical absorption region by using the hydrothermal and microwave processes. The hydrothermal treatment of dicyandiamide will enable it to transform into the melamine-cyanaurate complex which would then form the ultrathin CN nanosheets with the abundant porous structure. And then the microwave-assisted thermolysis process helps the generation of g-C3N4 which retains the highly crystalline and the enhanced optical absorption region. Eventually, the highly crystalline network comes with the narrowed band gap and the higher specific surface area encounter in optimized g-C3N4 leading it exhibits a relatively high photocatalytic performance than pristine bulk carbon nitride which the hydrogen evolution rate of it reaching 81.6 mu mol/h.