The invention discloses a copper indium gallium selenium thin film solar cell based on high quality prefabricated copper layer in uniform distribution and a preparation method thereof. The preparation method comprises the steps of preparing a molybdenum back electrode on a substrate and carrying out preprocessing on the molybdenum back electrode, and orderly preparing an absorption layer, a buffer layer, an intrinsic zinc oxide layer, an aluminum-doped zinc oxide layer, and upper electrode on the molybdenum back electrode which is subjected to preprocessing. The step of preparing a molybdenum back electrode on a substrate and carrying out preprocessing on the molybdenum back electrode comprises the steps of (1) using a sputtering method to deposit the molybdenum back electrode on the substrate, (2) cleaning the surface of the substrate deposited with the molybdenum back electrode, washing the substrate by using deionized water, and drying the substrate by using nitrogen, and (3) soaking the processed substrate in the step (2) into a corrosion liquid to carry out ultrasonic corrosion, washing the substrate by using deionized water, and drying the substrate by using nitrogen. The prefabricated copper layer prepared by the invention has the advantages of smooth surface and small roughness, prepared copper indium gallium selenium thin film grains have the advantages of a consistent size and uniform distribution, and the prepared copper indium gallium selenium thin film solar cell has the advantages of good uniformity and high conversion efficiency.