The invention discloses a extra highly substituting cellulose carboxymethyl sodium, which comprehensively utilize spraying impregnation and high-temp mist agitation. Charging a proportion of alkali and cellulose material sequentially into complex solvent of isopropanol, ethanol and water to carry out invasive quaternization, fractionally spray residual alkali liquid in mist into the reaction system by pressurization under high-temp agitation, charging etherifying agent to execute etherification for four times, and purifying to get final product. The product is high in degree of substitution, whose substituent distribute equally, has characteristics of high viscosity, high content of sodium, high acid-viscosity ratio. Meanwhile, the product has great heat resistance, great pseudoplastic behavior, and excellent suspension stability, prevent generation of gel particles, can have cooperative synergism with other stabilizer. The product is reduced in dosage, with improved taste and prolonged shelf life.