Provided is a composition for preparing plant fiber composite has, by weight based on the whole composition: a plant fiber raw material in an amount ranging from 60% to 80%; a starch auxiliary in an amount ranging from 10% to 30%; and a biological polymer additive in amount ranging from 10% to 20%, wherein the biological polymer additive includes a plant hormone, an enzyme, a vinegar and an ester of lactic acid. Plant fiber composite prepared from the composition is also provided. The plant fiber composite is derived from recycled material for extrusion and modeling, as a non-toxic and harmless raw material with high utilizing rate of recycled material and substitutes for plastic materials. The plant fiber composite is applied to food container, packing materials for electronic device and agricultural products, seedling trays, or substitutes for timber in the field of building, decoration and furniture.