The invention discloses a method of preparing a modified polysaccharide tanning agent, and is characterized that polysaccharide which is 5 to 10 parts by weight and water which is 2 to 20 parts by weight are activated for 5 to 30 minutes through ultrasonic the power or which is 1000W, are filtered and dried to obtain activated polysaccharide the accessibility of which is increased; the activated polysaccharide which is 5 to 10 parts by weight, water which is 2 to 20 parts by weight, sodium hypochlorite which is 10 to 200 parts by weight, and sulfuric acid which is 1 to 10 parts by weight are evenly mixed and react for 2 to 6 hours under the temperature ranging from 30 to 60 DEG C; then sodium hydroxide which is 5 to 100 parts by weight is added in, and the mixed solution is evenly stirred, filtered and washed; then water which is 2 to 20 parts and periodic acid which is 5 to 10 parts are added, and an electric reaction is performed for 2 to 6 hours under the separation of a cationic membrane, wherein, the pressure is 5 to 10V, and the current is 25 to 50mA, the mixed solution is filtered, washed, and dried to obtain the modified polysaccharide tanning agent with aldehyde groups and carboxyl groups. In leather pale yellow which is tanned by the modified polysaccharide tanning agent, the grain tightness is smooth, the contraction temperature ranges from 80 to 85 DEG C, and the content of aldehydes in leathers are less than 40mg/kg in accordance with national standards.