The rational design of the directional charge transfer channel represents an important strategy to finely tune the charge migration and separation in photocatalytic CO2‐to‐fuel conversion. Despite the progress made in crafting high‐performance photocatalysts, developing elegant photosystems with precisely modulated interfacial charge transfer feature remains a grand challenge. Here, a facile one‐pot method is developed to achieve in situ self‐assembly of Pd nanocrystals (NYs) on the transition metal chalcogenide (TMC) substrate with the aid of a non‐conjugated insulating polymer, i.e., branched polyethylenimine (bPEI), for photoreduction of CO2 to syngas (CO/H2). The generic reducing capability of the abundant amine groups grafted on the molecular backbone of bPEI fosters the homogeneous growth of Pd NYs on the TMC framework. Intriguingly, the self‐assembled TMCs@bPEI@Pd heterostructure with bi‐directional spatial charge transport pathways exhibit significantly boosted photoactivity toward CO2‐to‐syngas conversion under visible light irradiation, wherein bPEI serves as an efficient hole transfer mediator, and simultaneously Pd NYs act as an electron‐withdrawing modulator for accelerating spatially vectorial charge separation. Furthermore, in‐depth understanding of the in situ formed intermediates during the CO2 photoreduction process are exquisitely probed. This work provides a quintessential paradigm for in situ construction of multi‐component heterojunction photosystem for solar‐to‐fuel energy conversion. Non‐conjugated insulating polymer encapsulation and in‐situ growth of metal nanocrystal synergistically contribute to the spatially separated bi‐directional charge transfer pathways toward boosted solar CO2‐to‐syngas conversion.