A method for electrochemically converting a carbon dioxide gas into expected products includes using a member reactor. In the method, a membrane reactor includes a cavity, a solid electrolyte membrane separator, a cathode, an anode, and a fuel cell is provided. A cathode electrolyte and the carbon dioxide gas are passed through the cathode, and an anode electrolyte and an anode active material are passed through the anode chamber at the same time. An electrolytic voltage is applied to decompose the carbon dioxide gas into expected products. The expected products include a hydrogen gas and an oxygen gas which are fed back to the fuel cell to generate electric power.