The use of gas diffusion electrode (GDE) based flow cell can realize industrial-scale CO₂ reduction reactions (CO₂RRs). Controlling local CO₂ and CO intermediate diffusion plays a key role in CO₂RR toward multi-carbon (C₂₊) products. In this work, local CO₂ and CO intermediate diffusion through the catalyst layer (CL) was investigated for improving CO₂RR toward C₂₊ products. The gas permeability tests and finite element simulation results indicated CL can balance the CO₂ gas diffusion and residence time of the CO intermediate, leading to a sufficient CO concentration with a suitable CO₂/H₂O supply for high C₂₊ products. As a result, an excellent selectivity of C₂₊ products ~ 79% at a high current density of 400 mA·cm⁻² could be obtained on the optimal 500 nm Cu CL (Cu500). This work provides a new insight into the optimization of CO₂/H₂O supply and local CO concentration by controlling CL for C₂₊ products in CO₂RR flow cell.