The flow-type artificial photosynthesis converts CO₂ and H₂O into multi-hydrocarbons (C₂₊), combining both environmental and economic benefits. However, it is limited by the low C₂₊ selectivity and low CO₂ conversion efficiency. In this work, we coupled the photovoltaic hydrogen production from water and the photothermal CO₂ hydrogenation system to form a new artificial photosynthesis system, exhibiting a CO₂ conversion rate of 29.3% with C₂₊ selectivity reaching 82.1% at ambient pressure and 1 kW·m⁻² of sunlight irradiation, which is beyond the state of the art of traditional artificial photosynthesis. The excellent sunlight driven C₂₊ generation performance is attributed to the catalyst of alkali metal K doped Fe/Fe₃C, which is synthesized by Berlin green (BG) pyrolysis by controlling the BG precipitation aging temperature. The heterostructure of K doped Fe/Fe₃C can promote the formation of C₂₊ by enhancing CO₂ adsorption/activation and promoting C–C coupling at the interface of Fe and Fe₃C. This study provides new insights for designing efficient CO₂ conversion catalysts and is of great significance for promoting the practical application of artificial photosynthesis technology.