The invention belongs to the technical field of nano-material synthesis, and particularly relates to a preparation method and the application of a ternary composite photocatalyst. The preparation method comprises the following steps: first, synthesizing CQDs (Carbon Quantum Dots) with a remarkable up-conversion fluorescence effect by utilizing a simple one-step alkali auxiliary ultrasonic processing method; then, generating silver oxide (Ag2O) with an octahedral structure in CQDs solution by a wet-chemical method to obtain a CQDs/Ag2O compound with a CQDs coating layer; finally, loading silver (Ag) nano-particles on the surface of the CQDs/Ag2O by using chemical deposition and photo-deposition methods. The ternary composite photocatalyst can be used for degrading organic pollutants, such as rhodamine B, under full spectrum.