Heat treatment technology is usually used for the processing of metal materials. In this paper, polymer carbon nitride (g-C3N4) is secondarily calcined at 590 degrees C. A series of nitrogen-doped g-C3N4 are obtained by adjusting the holding time. The doping of nitrogen atoms changes the electronic structure of the catalyst and narrows the band gap. In addition, the escape of ammonia during the secondary calcinations leads to the increase in the specific surface area of the catalyst, which is helpful to enhance the photosensitization effect. Among them, the sample obtained by holding time at 2 h has the highest photogenerated carrier separation efficiency, the largest specific surface area and the best photocatalytic degradation of methylene blue.