The invention discloses a completely degraded plant powder modified thermoplastics composite material and preparation method thereof, which consists of the components with the weight portions as follows: 100 portions of degradable thermoplastics resin, 1-400 portions of plant powder, 0.1-40 portions of dispersing agent, 0.5-60 portions of impact modifier, 0-40 portions of inorganic filler as wellas 0.1-10 portions of stabilizing agent. The degraded thermoplastics resin has similar physiochemical performances with universal polyethylene and polypropylene, has excellent processing performances, and can be commixed with plant powder to prepare thermoplastics wood plastic composite materials. The obtained thermoplastics wood plastic composite materials can be completely degraded into carbon dioxide and water in natural environment under the effects of broadly existed microbes like germs, actinomycetes, etc. Compared with wood products, the composite material has the characteristics of good durability and dimensional stability, easy forming, small water absorbability, and corrosion resistance; compared with plastic, the composite material has the characteristics of low cost and high rigidity. The completely degraded plant powder modified thermoplastics composite material can be used for producing the shells of electric apparatuses like TV, sound, copier, printer, etc, and can alsobe applied on other workpieces used for injection.