Bimetallic　alloy　of　CuPd　nanoclusters　decorated　SnNb2O6　nanosheet　(CuPd/SN)　as　a　multifunctional　photocatalyst　has　been　constructed　successfully.　Compared　with　SN,　CuPd/SN　achieves　an　excellent　performance　for　semihydrogenation　of　phenylacetylene　under　visible　light　irradiation.　Especially,　CuPd/SN　with　the　Cu:Pd　molar　ratio　of　3:1　exhibits　the　highest　catalytic　conversion　efficiency　for　phenylacetylene　(99.6%)　with　high　selectivity　of　phenylethylene　(99.4%).　In　situ　FTIR　result　reveals　that　SN　could　activate　phenylacetylene　via　a　C　equivalent　to　C　-＞　Nb　pi-bonded　coordination　which　improves　the　selectivity　of　phenylethylene.　Experiment　results　indicate　that　atom　Pd　in　alloy　is　responsible　for　dissociation　H2,　while　Cu　represses　the　overhydrogenation.　Electrochemical　measurements　suggest　that　CuPd　alloy　on　SN　surface　could　greatly　minimize　the　recombination　of　photogenerated　electron　-　hole　pairs.　The　separated　electrons　further　facilitate　the　production　of　active　H.　Hydrogen　spillover　would　transfer　active　H　to　Nb　site　for　phenylacetylene　semihydrogenation.　Finally,　a　synergetic　catalytic　mechanism　is　deeply　discussed　at　molecule　scale.　This　work　would　provide　a　deep　insight　for　designing　a　multifunctional　photocatalyst.