Transition　metal　chalcogenides　(TMCs)　with　a　large　absorption　coefficient,　a　favorable　energy　level　position,　and　abundant　active　sites　have　attracted　considerable　attention　in　heterogeneous　photocatalysis.　However,　the　photocatalytic　redox　activities　of　TMCs　are　hindered　by　rapid　charge　recombination,　sluggish　charge　transport　kinetics,　and　poor　stability.　Herein,　a　TMC-based　heterostructure　artificial　photosystem　is　fabricated　by　a　controllable　cation　exchange　strategy　under　ambient　conditions,　wherein　partial　substitution　of　Cd2+　with　Ag+　ions　over　a　CdS　substrate　enables　the　epitaxial　growth　of　an　ultrathin　Ag2S　layer　on　the　CdS　surface,　ultimately　giving　rise　to　the　CdS/Ag2S　heterostructures.　The　thus-obtained　CdS/Ag2S　heterostructure　demonstrated　markedly　enhanced　photoredox　catalytic　activities　in　the　anaerobic　photoreduction　of　aromatic　nitro　compounds　to　amino　derivatives　and　photocatalytic　mineralization　of　organic　pollutants　under　visible-light　irradiation,　far　surpassing　the　pristine　CdS　counterpart,　primarily　owing　to　the　applicable　energy　level　configuration　formed　between　CdS　and　Ag2S　as　well　as　the　favorable　interfacial　integration.　The　synergy　facilitates　spatially　vectorial　charge　separation　and　substantially　prolongs　the　charge　lifetime　of　CdS/Ag2S　heterostructures.　Our　work　would　spark　interest　in　exploring　diverse　TMC-based　heterostructures　via　cation　exchange　for　solar　energy　conversion.