Membrane electrode assembly (MEA) is widely considered to be the most promising type of electrolyzer for the practical application of electrochemical CO2 reduction reaction (CO2RR). In MEAs, a square-shaped cross-section in the flow channel is normally adopted, the configuration optimization of which could potentially enhance the performance of the electrolyzer. This paper describes the numerical simulation study on the impact of the flow-channel cross-section shapes in the MEA electrolyzer for CO2RR. The results show that wide flow channels with low heights are beneficial to the CO2RR by providing a uniform flow field of CO2, especially at high current densities. Moreover, the larger the electrolytic cell, the more significant the effect is. This study provides a theoretical basis for the design of high-performance MEA electrolyzers for CO2RR.