The invention provides a preparation method of degradable modified polyethylene plastic for packaging and belongs to the field of plastic, in particular to the preparation method of the degradable modified polyethylene plastic. The preparation method mainly includes the steps that under a high pressure condition, modified starch, EVA, hydroxy propyl cellulose and Pulullan are utilized to prepare modified polyethylene, then the modified polyethylene is added to a mixture of polyethylene alcohol, starch, dicumyl peroxide, a silane coupling agent and octodecane amide, and the degradable modified polyethylene plastic is prepared in a mixed mode in a twin-screw extruder at a high temperature. A prepared polyethylene plastic product has extremely good transparency and glossiness, is free of dust collection, free of static electricity, good in printing adaptability, high in tensile strength and elongation, good in wear resistance, extremely good in air barrier property and fragrance maintenance, excellent in water resistance and degradability and particularly suitable for being used for packaging materials.