The invention relates to a brightening agent, especially to a plastic brightening agent. The plastic brightening agent comprises, by mass, 10 to 20 parts of waterborne polyurethane, 40 to 60 parts of paraffin, 30 to 50 parts of N,N'-ethylidenebisoleamide, 20 to 40 parts of N,N'-ethylenebisoleamide, 5 to 10 parts of polyoxyethylene amide, 12 to 18 parts of a cosolvent and 150 to 180 parts of a solvent. The plastic brightening agent provided by the invention has a high molecular weight and a melting point of 200 DEG C; and the plastic brightening agent can form a film on the surface of a plastic product so as to improve the brightness and smoothness of the surface of the plastic product. Compared with conventional plastic brightening agents, the plastic brightening agent provided by the invention has the following advantages: 1, brightening efficiency is high and long-lasting, and the plastic brightening agent cannot be easily wiped and forms the film on the surface of the plastic product; 2, the plastic product is not reduced in its physical properties and secondary processability; 3, the plastic brightening agent has excellent thermal and chemical stability and is not decomposed and not volatile during processing; 4, the plastic brightening agent does not corrode equipment or contaminate products; and 5, the plastic brightening agent is nontoxic.