The present invention provides a composition, a gypsum board and their preparation method and the use of ascorbic acid as an anti-sagging additive in a gypsum board. Said gypsum board comprises set gypsum prepared from the composition; while said composition comprises hemi-hydrate gypsum, water and a compound comprising group and no carboxylate groups. The set gypsum prepared from the said composition shows better anti-sagging or deformation resisting property. The gypsum board 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 board to meet the demands of the customer.