Solar-driven CO2 conversion to chemical fuels in an aqueous solution is restricted not only by photocatalysts but also by mass transfer. Here, a regulatable three-phase interface on a porous fixed-bed is constructed for efficient C–C coupling in photocatalytic CO2 reduction. The photocatalytic results show that ∼90% selectivity towards C2+ products is obtained by a Cu/Cd0.5Zn0.5S photocatalyst, with a yield of 6.54 ​μmol/h (an irradiation area of 0.785 cm2), while only 0.94 ​μmol/h (an irradiation area of 19.625 cm2) is achieved with a commonly used suspension photocatalytic reactor. We find that under the same CO2 feed rate, the local CO2 concentration in this porous fixed-bed photoreactor is obviously higher than in the suspension photoreactor. The larger local CO2 coverage derived from a higher CO2 supply and aggregation enhances the C–C coupling, thereby generating more C2+. Even an observable three-phase interface on the porous fixed-bed can be regulated by adjusting the CO2 supply, for which the optimal gas inlet rate is 5–10 sccm. Display omitted •A homemade porous fixed-bed photoreactor for overcoming the mass transfer bottleneck was designed and constructed.•A three-phase interface was formed and regulated by adjusting CO2 feed, while a K+ tracer experiment was used to observe it.•Local CO2 concentration from the three-phase interface enhanced ∗CO–COH coupling, with ∼90% selectivity to C2+ products.