Graphitic carbon nitride (g-C3N4), as a metal-free green semiconductor material, is becoming one of the popular materials in the field of photocatalysis gradually, and shows a broad development prospect and application potential in degrading environmental pollutants. The mechanism of degradation of pollutants by g-C3N4 photocatalyst is analyzed, and the studies on photocatalytic modification of g-C3N4 are also described in terms of morphological structure design, metal/non-metal doping, and construction of heterojunction. The application studies of g-C3N4 photocatalysts for pollutant degradation in three different environments, namely, gas, soil and water are summarized. Finally, the practical application of g-C3N4 in the field of packaging is prospected.