The invention belongs to the technical field of biodegradable materials, and discloses a water-soluble porous completely biodegradable composite material for preparing a seedling container and a preparation method thereof, and a seedling container. The composite material is composed of the following components in percentage by weight: 10-50% of polybutylene adipate terephthalate copolymer, 5-20% of polylactic acid, 20-50% of thermoplastic starch, 20-50% of modified bagasse, 1-15% of a water-soluble pore-foaming agent, and 0-5% of other auxiliaries; the modified bagasse is baggage modified witha modifier, the modifier is at least one of gamma-aminopropyltriethoxysilane, gamma-glycidyloxypropyltrimethoxysilane and maleic anhydride, and the amount of the modifier is 0.5-2% of the mass of thebagasse. The invention also provides a seedling container based on the composite material, which can be applied to seedlings of flowers, Chinese medicinal materials, fruits, vegetables, and the like,and effectively improves the production and cultivation effects of crops.