Abstract: A preparation method for boron-free aldehyde-free modified polysaccharides includes: deionized water, polar organic compounds, low molecular weight sugars, monovalent metal salts and polysaccharides are evenly dispersed; add catalyst; raise the temperature to 45° C.-120° C., activate for 30-600 minutes, cool and filter; etherifying agent is added to the filter material to heat up the reaction; cool down to below 40° C., add acid to adjust pH to near neutral, filter; the modified polysaccharides are washed and purificated by adding solvent, and then filtered and dried to obtain the target product.