The invention relates to a method for preparing bacterial cellulose by controlling a molecular weight. During the preparation process of the bacterial cellulose, a culture system is optimized, and glyceraldehydes or mannose is directly used as an end-capping carbon source to be synthesized into a product molecule, so that the product is provided with no structure allowing the re-growth of a cellulose molecular chain; and therefore, a purpose of controlling the growth of the molecular chain is realized, the molecular weight is controlled, and the dissolvable BC of different molecular weights is prepared. By adopting the preparation method, the molecular weight of the bacterial cellulose is greatly reduced; and meanwhile, the product is relatively good in a microstructure, yield, water retention rate, crystallization index, heat stability and reproducibility, the operation is simple and convenient, and the industrialized production is facilitated.