The present invention discloses a method for preparing a lightweight thermal-insulating board from alkali-gelatinized starches used as adhesive, waste plant fibers and polystyrene foams. The method comprises the followings steps: dropwise adding sodium hydroxide with certain concentration into a starch milk so as to obtain an alkali-gelatinized starch adhesive; then evenly mixing the obtained alkali-gelatinized starch adhesive with the waste plant fibers and the polystyrene foams, and putting the obtained mixture into a mold; carrying out forming and drying on the mixture so as to obtain the lightweight thermal-insulating board. The method disclosed by the present invention is simple in production process and easy to operate; and the lightweight thermal-insulating board prepared by using the method disclosed by the invention does not contain the harmful materials such as formaldehyde, therefore, the lightweight thermal-insulating board is an environmental-friendly lightweight thermal-insulating board.