Graphitic carbon nitride (g-C₃N₄) is viewed as a promising visible-light photocatalyst for industrialization due to its low processing temperature and high chemical stability. However, serious charge recombination caused by incomplete polymerization during direct calcination of nitrogen-rich precursors significantly limits its photocatalytic performances. To boost charge separation, herein, we propose a rational strategy by constructing a crystalline g-C₃N₄/g-C₃N_4−xS_x isotype heterostructure through the molten salt method. Theoretical calculation reveals that apparent charge-transfer channels are formed between g-C₃N₄ and S-doped g-C₃N₄ layers in the heterostructure. Owing to high crystallinity for decreasing charge recombination and isotype heterostructure for efficient charge transfer, the as-prepared g-C₃N₄/g-C₃N_4−xS_x showed remarkable photocatalytic performances with the hydrogen production rate elevated by up to 12.3 times of its singular components. Another novelty of this work is we investigated for the first time the piezocatalytic activity of crystalline g-C₃N₄ by characterizing its performance for H₂O₂ generation and KMnO₄ reduction. Strikingly, its superior piezocatalytic performance over components can be further improved by NaBH₄ treatment, which is uncovered to enhance the asymmetric structure of crystalline g-C₃N₄ by introducing extra cyano groups and removing partial NH_x species in its tri-s-triazine layer structure. This work opens up new strategies for the design of highly efficient polymeric photocatalysts and highlights the piezocatalytic studies of g-C₃N₄.