Treat the refined cotton cellulose with alkali solution at 35-40°C for half an hour, press, pulverize the cellulose, and properly age at 35°C, so that the average degree of polymerization of the obtained alkali fiber is within the required range. Put the alkali fiber into the etherification kettle, add propylene oxide and methyl chloride in turn, and etherify at 50-80℃ for 5h, the maximum pressure is about 1.8MPa. Then add an appropriate amount of hydrochloric acid and oxalic acid to the hot water at 90 ° C to wash the material to expand the volume. Dehydrate with a centrifuge, wash until neutral, when the water content in the material is less than 60%, dry it with a hot air flow at 130°C until the content is less than 5%, and finally pulverize and pass through a 20-mesh sieve to obtain the finished product.