The invention provides a preparation method for modified mixed cellulose ether. The preparation method comprises the steps of alkalization, etherification, desolventizing acid modulation and aftertreatment. According to alkalization, alkali liquor is adopted for carrying out alkali activation treatment on cellulose powder and starch; according to etherification, chloromethane, epoxypropane and ethylene oxide are adopted as an etherifying agent for etherifying the alkalified cellulose powder and starch for 90-120 minutes at the temperature of 30-54+/-1 DEG C, 65-80+/-1 DEG C and 85-95+/-1 DEG C; according to desolventizing acid modulation, acid and hot water are added firstly, desolventizing is carried out, the desolventizing temperature ranges from 80 DEG C to 90 DEG C, and the pH value of the material obtained after desolventizing ranges from 5 to 7; the modified mixed cellulose ether is prepared from, by weight, 80-95 parts of cellulose powder, 1-3 parts of starch, 58-100 parts of chloromethane, 18-25 parts of epoxypropane and 8-10 parts of ethylene oxide. The preparation method is short in production period, and the obtained product has excellent water-retaining performance, dispersity, solubility, film-forming performance, stability, anti-mildew performance, ptosis resistance and high crushability. The invention further provides mixed modified mixed cellulose ether obtained through the preparation method.