In order to improve the photocatalytic ability of graphite-phase carbon nitride, Er was selected as a precursor to modify graphite-phase carbon nitride, and the graphite-phase carbon nitride composite (Er/g-C3N4) doped with rare earth (Er) was prepared by thermal polymerization. The results showed that the 0.5% Er/ g-C3N4 composite had strong visible light absorption. The results showed that the 0.5% Er/ g-C3N4 catalyst had a good visible light absorption and the 0.5% Er/ g-C3N4 catalyst had a better degradation effect on RhB.