The invention relates to a combined process for separating hydrogen and carbon dioxide from shift gas produced by a dry gas hydrocarbon steam conversion method, separating components in the shift gas by combining wet-method decarburization and pressure swing adsorption and performing circular backflow on impurity gas subjected to desorption by pressure swing adsorption denitrification gas. By the combined process, high-purity hydrogen and carbon dioxide are obtained, the recovery rate of the hydrogen can be improved, and the pressure of a desorption tower is improved by reducing the recovery rate of CO2, so that the pressure of CO2 is improved, and the compression cost of recovering CO2 is reduced.