A membrane reactor used for electrochemically converting a carbon dioxide gas into an expected product includes a cavity, a solid electrolyte membrane separator, a cathode, an anode, and a power source. The solid electrolyte membrane separator is disposed in the cavity and divides the cavity into two chambers defined as a cathode chamber and an anode chamber. The cathode is disposed in the cathode chamber, and the anode is disposed in the anode chamber. The cathode is a trickle bed structure including a porous conductive layer and cathode particles disposed on the porous conductive layer. The power source is disposed outside the cavity to provide an electrolytic voltage.