As a complex system involving three phases, the performance of CO2 electroreduction (CO2ER) is commonly constrained by mass transfer limitations originated from the low solubility of CO2 in aqueous electrolytes. Although some work has been carried out on mass transfer limitation, the underlying mechanism remains ambiguous. In this paper, a mass transfer model based on the Nernst-Planck equation was established to provide species concentration profiles within the catalyst layer and the internal effectiveness factor η was calculated to quantify the severity of mass transfer restrictions. The effects of catalyst layer thickness, diffusion layer thickness, and buffer concentration on CO2ER were evaluated based on η. Results show that the internal mass transfer is obvious in the catalyst layer. Increasing catalyst layer thickness only has a marginal benefit on current density — a thin catalyst layer (of about 15 μm) can give an excellent performance under an acceptable mass transfer limitation.