The invention discloses a preparation method of a plant polyphenol formaldehyde catching agent. The preparation method comprises the steps that fresh tea leaves are frozen at low temperature, dried and dewatered; the tea leaves are broken to be in 10-20 meshes, the broken tea leaves are loaded in an extracting bag, the extracting bag is placed in a closed extracting tank, vacuumizing is performed until the closed extracting tank is under the pressure of minus 0.1 MPa, extracting is performed for 1-2 h by introducing 95% methyl alcohol, and then an extracting solution is obtained; the extracting solution is transferred into a vacuum concentrating tank, and reduced pressure distillation is performed at 60 DEG C to obtain a polyphenol concentrated solution; the polyphenol concentrated solution is dewatered and dried, the vacuum degree is controlled at minus 0.1 MPa, the temperature is controlled at 60 DEG C, and dry plant polyphenol powder is obtained; the dry plant polyphenol powder is dissolved in a 1% aqueous glycerol solution to prepare a stock solution; an assisted solution is prepared; the stock solution and the assisted solution are mixed according to the volume ratio of 1:1, and then the product is obtained. The preparation method is simple and easy to operate, low in preparation cost, free of secondary pollution, environmentally friendly, safe and efficient, plant polyphenol of extract is not destroyed and oxidized, and the biological activity of the extract is better retained.