The invention discloses a CNC/PEG composite coating. The CNC/PEG composite coating contains cellulose nanocrystals and polyethylene glycol, wherein the polyethylene glycol accounts for 0-50% of the mass percentage content of the cellulose nanocrystals, the particle length dimension of the cellulose nanocrystals is 151-167 nm, and the diameter is 48-52 nm. The invention further discloses a preparation method of the CNC/PEG composite coating. Natural cellulose is utilized as a raw material, the non-toxic and pollution-free PEG is utilized as a coating color regulator and a plasticizer, the prepared colorful coating is environmentally friendly and is degradable, the mechanical properties are greatly improved, and a structural color is bright and can be adjusted and controlled. In addition, the coating can be prepared through blending and compounding and dropwise simple coating on the surface of a base material, the preparation process is simple and easy to control, and the cost is low.