The invention provides a method for preparing a flame-retardant building panel from straws. The method comprises the following steps: carrying out crushing, sorting, dust removal and drying on the straws so as to obtain a crushed straw aggregate, dipping and wetting the crushed straw aggregate by virtue of an inorganic salt solution, so as to obtain a wet straw aggregate; stirring and mixing the obtained wet straw aggregate with an inorganic gel material and an inorganic auxiliary, so as to obtain a mixture; and uniformly spreading the obtained mixture in a mold frame with a specific size, carrying out compact formation, removing the mold frame, and carrying out solidifying formation on a panel blank by virtue of a hot-pressing process or carrying out normal-temperature standing maintenance to expected strength, so as to obtain the finished product panel. According to the method, the inorganic gel material is taken as an adhesive, and the product does not release pollutants, so that the indoor environment is protected, and the quality of a living environment of people is improved.