Graphite carbon nitride (g-C3N4) has become a research hotspot owing to its special electronic structure and excellent chemical stability. However, although g-C3N4 has made great progress in the field of photocatalysis, its high recombination rate and narrow visible light absorption range still restrict its photocatalytic efficiency. Herein, we focused on the previous works on g-C3N4 with different degree of polymerization (DP) distributions to explore its regulation mechanism, which is designed to enhance its photocatalytic performance by adjusting its crystal structure to an appropriate state with a narrow band gap and efficiently separate photoexcited electrons and holes. Specifically, the effect of the DP distribution/value on the photocatalytic performance of g-C3N4 is discussed from the perspective of polymerization kinetics and crystal structure engineering. Meanwhile, clarification of the breakthroughs in the crystallization properties of g-C3N4 has led to innovative and encouraging applications in environmental purification and energy transformation. Finally, some innovative remarks and perspectives on the challenges for regulating the DP distribution/value of g-C3N4 are presented.