A method for preparing alcohol by utilizing potato materials comprises the following steps: grinding the potato materials, mixing the products after grinding with enzyme and carrying out enzymolysis to obtain enzymolysis products; fermenting the enzymolysis products, separating alcohol from the fermentation products, carrying out solid-liquid separation on the liquid waste obtained by fermenting the potato materials to prepare the alcohol to obtain solid products and liquid products and carrying out wastewater treatment on the obtained liquid products. The method is characterized by adding water-soluble alginate esters and/or water-soluble alginates into the liquid waste before carrying out solid-liquid separation on the liquid waste. The method provided by the invention obviously shortens the solid-liquid separation time and increases the treatment capacity of the liquid waste in unit time under the condition of dispensing with new equipment, thus overcoming the bottleneck that the device used for preparing the alcohol by fermenting cassava has relatively low operation load and increasing the production capacity of the device.