The invention discloses a carbon-supported PtCu alloy catalyst with a nanopore structure and a preparation method thereof, and belongs to the water electrolysis-organic matter electrocatalytic reduction coupling technical field. The catalyst takes graphite fiber cloth as a carbon carrier, after the carbon carrier is subjected to ultrasonic cleaning under the normal pressure and subjected to ion beam sputtering auxiliary cleaning under a vacuum environment, a Pt target inserted with Cu is sputtered by a conventional ion beam, Pt and Cu active metal components are supported on the carbon carrier, and a carbon-supported PtCu alloy catalyst is obtained through a vacuum heat treatment method; and then the carbon-supported PtCu alloy catalyst is placed in an HNO3 solution and is soaked and subjected to acid corrosion, and the carbon-supported PtCu alloy catalyst with the nanopore structure is obtained though deionized water ultrasonic washing and vacuum constant-temperature drying treatment; the content of Pt is 0.148-0.151 mg/cm<2>, the open nanopore density is 6.25*10<10>-4.00*10<12> cm<-2>, the pore diameter is 5.0-40.0 nm, and the pore wall thickness is 1.0-2.0 nm. In the premise of low Pt content, the method increases the electrochemical activity specific surface area of the carbon-supported PtCu alloy catalyst, and then enhances the electrocatalytic activity.