In this work, two-dimensional crystalline C3N4 (CCN) was obtained by promoting the self-condensation reaction of dicyandiamide in an air atmosphere via the molten salt method. Studies show that compared with polymer-C3N4 (PCN) ((9.8 m2/g)), CCN has a larger specific surface area (77.1 m2/g)) and better crystallinity. Additionally, the resistance of CCN in the air is 40 MΩ, 1250 times lower than that of polymer-C3N4 (PCN) (∼50000 MΩ). Secondly, when 50 ppm toluene gas analysis flows into the CCN based gas sensor at room temperature, the gas response value is 350. The CCN based gas sensor works for 60 days, or after 25 cycles continuously, the gas response value remains at about 350 (Rg/Ra), which is due to the excellent crystallinity, optimized π conjugate system, and enhanced interlayer van der Waals of CCN. Additionally, the critical effect of crystallinity on the gas sensitivity of carbon nitride is also discussed in detail.