The invention relates to a biodegradable plastic biomass composite material and a preparation method thereof. The biodegradable plastic biomass composite material is prepared from the following raw materials in parts by weight: 30 to 50 parts of flour, 6 to 8 parts of lignocellulose, 10 to 20 parts of melamine resin, 5 to 20 parts of calcium carbonate, 20 to 50 parts of hydrogen peroxide retarderthinner, 3 to 7 parts of rosin derivative, 2 to 4 parts of vegetable oil, and 1 to 5 parts of antioxygen. According to the biodegradable plastic biomass composite material provided by the invention, alarge proportion of melamine resin and flour is added, the material contains a carbon source capable of being utilized by microorganisms, protein and other microelements, the microorganisms can be multiplied and survived on a residual body, and a certain of substances are secreted to gradually decompose a macromolecular material into fragments and turn into other food which is digested under theaction of soil fungi, so that a packaging box has a favorable degradation property.