New water-soluble cellulose derivatives containing quaternary ammonium groups were prepared by the heterogeneous reactions of hydroxyethyl cellulose with a quaternary chlorohydrin and characterized by reaction parameters and infrared analyses. The dependence of the heterogeneous reaction on the different affecting factors was studied. The cellulose derivatives obtained were investigated for their property as the water-soluble polymer for inhibiting the swelling of water-sensitive clay in oilfields. Based on the kinetic curves for clay hydration swelling, a mathematical model that relates linear clay expansion as a function of hydration rate and hydration equilibrium constant was developed.