The invention discloses a method for preparing polysaccharides-modified dispersant with controllable performance. The method comprises the following steps: firstly adding a modifier to the polysaccharide substance to prepare the modified polysaccharide; and mixing the modified polysaccharide with a monomer, a reducing agent, an initiator and water to carry out a radical polymerization to obtain the polysaccharide modified dispersant. By adjusting the process, the type and amount of the modifier, the method prepares the modified polysaccharides with different degradation degrees and different oxidation degrees, and then the modified polysaccharides and vinyl acid are subjected to grafted copolymerization to obtain the target product. By controlling the molecular weight, the oxidation degree, and the addition amount of the polysaccharide, the molecular weight and the number of functional groups of the final dispersant are further controlled, and furthermore, the dispersing property of the dispersant can be controlled. The dispersant has certain degradability and also reduces the cost. Compared with traditional dispersants, the dispersant provided in the invention has obvious advantages and broader application prospects.