The invention relates to a method for preparing ethanol by adopting a raw material containing cassava residue, wherein the method comprises the following steps: mixing the raw material containing thecassava residue and enzyme, enzymolyzing cellulose in the raw material containing the cassava residue to obtain an enzymolysis product, and fermenting the enzymolysis product, wherein the method of mixing the raw material containing the cassava residue and the enzyme comprises mixing the raw material containing the cassava residue in batch and the enzyme, the batch times are at least two times, and the amount of the raw material containing the cassava residue added in each batch is at most 60 percent of the weight of the total raw material containing the cassava residue. The method has the advantages that the method can improve utilization rate of the raw material containing the cassava residue, and the yield of the prepared ethanol is higher.