Abstract: Electrocatalytic carbon dioxide reduction reactions （CO₂RR） are one of the key technologies for addressing global climate change and achieving energy transition. By converting CO₂ into valuable chemicals, greenhouse gas emissions can be effectively reduced, and sustainable energy development can be promoted. Hydrogen evolution reactions （HER） often occur during CO₂RR, producing H₂ and CO to form syngas. The H₂ to CO ratio in syngas significantly influences the synthesis of downstream chemicals. Therefore, regulating the structure of the catalyst, such as its thickness and porosity, is an effective approach to achieving precise control of target products. Ni−N−C catalysts, with their high selectivity for CO products, demonstrate significant advantages in the electrocatalytic syngas production process, particularly within the syngas ratio range of 0.5 to 5. The study employed finite element numerical simulation methods to construct a two-dimensional membrane electrode assembly （MEA） model, investigating the effects of Ni−N−C catalyst thickness and porosity on mass transfer, CO₂ reduction reaction （CO₂RR）, hydrogen evolution reaction （HER） selectivity, and product concentration distribution within the MEA. Simulations were conducted for catalyst thicknesses ranging from 5 to 95 μm and porosities from 0.2 to 0.9. It was found that as the catalyst thickness increased from 65 μm to 95 μm, CO₂ diffusion gradually enhanced, limiting the increase in CO concentration and thereby affecting the syngas ratio. Within the voltage range of 2.5 V to 3.1 V, the syngas ratio remained stable between 1 and 5. On the other hand, increasing the catalyst porosity, particularly within the range of 0.2 to 0.9, helps alleviate the diffusion-induced rate limitation of CO₂, thereby enhancing the concentration of CO products. Within the low voltage range of 1.6 V to 2.2 V, syngas with a ratio of 0.5 to 1 can be produced. This study provides theoretical support for the design and optimization of catalysts in MEA systems, offering guidance for future industrial applications of electrocatalytic CO₂ reduction reactions.