The graphite-like carbon nitride (g-C3N4) was prepared by pyrolyzation at high temperature, and then composited with CQDs by hydrothermal method to obtain the g-C3N4/CQDs photo-catalyst. The as-prepared g-C3N4/CQDs was characterized by SEM, TEM, FT-IR, XRD, UV-Vis/DRS, XPS and N2 adsorption and desorption isotherms. Using Rhodamine B (Rh B) as simulated pollutant, the photo-catalytic activity and stability of g-C3N4/CQDs have been investigated under visible light. The results show that the absorption wavelength of g-C3N4/CQDs shifts to the visible light region. When the content of CQDs in composite is 1.5%, the obtained g-C3N4/CQDs photocatalyst has the best catalytic activity. The photocatalytic degradation rate of Rh B by g-C3N4/CQDs is 54.5%, and the correlation coefficient of chemical reaction kinetics is R2=0.9982. The degradation rate of g-C3N4/CQDs is 1.38 times higher than that (39.5%) of pure g-C3N4. After 4 cycles of use, the catalytic degradation rate remained above 50%. The photocatalytic mechanism of catalyst on Rh B was studied. The results show that h+, ·O2-, H2O2 and ·OH were the main active species in the photocatalytic process. The order of oxidation of the four species from strong to weak was: h+ > ·O2- > H2O2 > ·OH.