Crystallinity and crystal structure greatly influence the photocatalytic behavior of photocatalysts. Pristine g-C₃N₄ produced by traditional thermal-induced polycondensation reaction bears low crystallinity and thus poor photoactivity, which originates from the incomplete polymerization of the precursor containing amine groups, abundant hydrogen bonds, and unreacted amino, as well as cyanide functional groups in the skeleton. During photocatalytic process, these residual functional groups often work as electron trap sites, which may hinder the transfer of electrons on the plane, resulting in low photoactivity. Fortunately, crystalline carbon nitride (CCN) was reported as a promising photocatalyst because its increased crystallinity not only reduces the number of carriers recombination centers, but also increases charge conductivity and improves light utilization due to extended π-conjugated systems and delocalized π-electrons. As such, we summarize the recent studies on CCN-based photocatalysts for the photoactivity enhancement. Firstly, the unique structure and properties of CCN materials are presented. Next, the preparation methods and modification strategies are well outlined. We also sum up the applications of CCN-based materials in the environmental purification and energy fields. Finally, this review concerning CNN materials ends with prospects and challenges in the obtainment of high crystallinity by effective techniques, and the deep understanding of photocatalytic mechanism.