This　work　reports　the　rapid　and　effective　photocatalytic　dechlorination　of　3,3＇,4,4＇-trtrachlorobiphenyl　(PCB77)　with　highly　dispersed　Pd　nanoclusters　loaded　two-dimensional　(2D)　TiO2(B)　nanosheets　(Pd/TiO2(B))　under　visible　light　irradiation,　achieving　a　dechlorination　efficiency　of　100　%　with　H2O　used　as　an　H-donor　source.　The　ultra-thin　TiO2(B)　nanosheets　provide　a　good　platform　for　studying　the　dechlorination　intermediates/pathways　of　PCB77　at　the　molecular　level.　It　is　proved　that　the　dechlorination　of　PCB77　is　a　stepwise　process,　in　which　chlorines　at　the　para　positions　are　much　more　susceptible　than　those　at　the　meta　positions　due　to　the　activation　of　the　C-Cl　bond　in　the　presence　of　Pd　nanoclusters.　Furthermore,　in-situ　FTIR　results　of　D2O　absorption　suggest　that　the　TiO2(B)　nanosheets　with　abundant　surface　defects　greatly　promote　the　activation　of　water　molecules　via　the　formation　of　surficial　complexes　by　hydrogen　bonds.　Finally,　a　synergistic　mechanism　is　proposed　for　photocatalytic　dechlorination　of　PCB77　over　Pd/TiO2(B).　The　study　provides　an　efficient　method　for　complete　dechlorination　of　PCB77　and　an　in-depth　understanding　photocatalyzed　dechlorination　reaction　mechanism　at　the　molecular　level,　which　may　extend　to　other　photocatalytic　dehalogenation　reactions　of　halogenated　pollutants.