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.　©　2023　Wiley-VCH　GmbH.