The invention provides high-density bamboo powder modified polylactic acid biodegradable plastic and a preparation method thereof. The high-density bamboo powder modified polylactic acid biodegradableplastic provided by the invention comprises the following components in parts by mass: 35-60 parts of polylactic acid, 25-40 parts of modified bamboo powder, 10-15 parts of starch, 1.5-2.5 parts of silicate, 3-5 parts of glycerol and 0.5-2 parts of a coupling agent. According to the technical scheme, the bamboo powder cellulose is subjected to graft modification treatment, wherein the N, N-dimethylformamide / aluminum chloride mixed system is an initiator for graft copolymerization reaction of bamboo powder cellulose and caprolactone; hydrophilic, oleophylic and incompatible two chain segmentsare bonded together by adopting a graft copolymerization method, and the modified bamboo powder is easily crosslinked with polylactic acid under the action of a coupling agent to form a blend, so thatthe mechanical property of the bamboo-plastic composite material is improved, the use amount of polylactic acid is further reduced along with the increase of the addition amount of the bamboo powder,and the cost is saved.