The present invention provides a gypsum composition, a gypsum board and their preparation method and the use of DHA as an anti-sagging additive in a gypsum product. Said gypsum board comprises set gypsum prepared from the composition; while said composition comprises gypsum, and an anti-deformation additive, wherein the anti-deformation additive comprises at least one selected from a group consisting of dehydroascorbic acid, dehydroascorbate and semidehydroascorbic acid. The set gypsum prepared from the said composition shows better anti-sagging or deformation resisting property. The gypsum product of the present invention is hardly distorted and has stronger stability even in the condition of high humidity, thus improve the quality of the gypsum product to meet the demands of the customer.