Polymer carbon nitrides (PCN) are one of promising photocatalysts in water splitting, but their low crystallinity usually leads to slow transfer efficiency of charge carriers. Herein, we presented a solid-salt-assisted growth strategy to fabricate highly crystalline polymer carbon nitride (CPCN). The high-melting-point KC1 salt was used as an easily removable solid template, where the confined space guides efficient growth of CPCN. The fabricated CPCN samples show high crystalline quality with clear long-range periodicity and large grain sizes up to 400 nm. The combination of structural characterization analyses and first-principles energetic calculations reveals that CPCN is composed of chain-like melon motifs with intercalated K+ ions. The photocatalytic activity of CPCN in hydrogen evolution reaction is over 22 times higher than that of pristine PCN, the apparent quantum efficiency at lambda = 420 +/- 5 nm is up to 11.4%. This work provides a simple and general protocol for crystallinity and performance enhancement in a wide range of polymeric photocatalyst materials.