Purpose　The　purpose　of　this　paper　is　to　investigate　the　diffusion　behaviors　of　different　atoms　at　the　Cu/Cu3Sn　interface　and　the　vacancy　formation　energy,　diffusion　energy　barrier　and　vacancy　diffusion　activation　energy.　Design/methodology/approach　The　diffusion　behaviors　of　different　atoms　at　the　Cu/Cu3Sn　interface　are　analyzed,　and　the　vacancy　formation　energy,　diffusion　energy　barrier　and　vacancy　diffusion　activation　energy　are　obtained　using　molecular　dynamics　simulation.　The　nudged　elastic　band　method　is　used　to　evaluate　diffusion　energy　barrier　for　Cu/Cu3Sn　system.　Findings　It　is　found　that　the　vacancies　in　the　Cu/Cu3Sn　interface　promote　the　interfacial　diffusion,　and　the　formation　energy　of　Cu　vacancy　in　the　Cu　crystal　is　larger　than　that　in　Cu3Sn　crystal.　In　addition,　the　formation　energies　of　Cu1　vacancy　and　Cu2　vacancy　are　close　to　each　other　in　Cu3Sn　crystal,　and　they　are　all　less　than　the　formation　energy　of　Sn　vacancy.　Furthermore,　the　vacancy　diffusion　barrier　and　vacancy　diffusion　activation　energy　of　the　Cu/Cu3Sn　interface　are　calculated,　and　the　results　show　that　the　vacancy　diffusion　activation　energy　of　Sn　was　higher　than　that　of　Cu.　Finally,　by　comparison　of　diffusion　activation　energies　of　different　diffusion　mechanisms,　Cu　-＞　Cu1vac　is　the　most　possible　migration　path　at　all　temperatures.　Originality/value　It　is　concluded　that　the　vacancies　in　Cu/Cu3Sn　interface　promote　interfacial　diffusion,　and　the　activation　energy　of　vacancy　diffusion　in　most　diffusion　mechanisms　decreases　with　the　increase　of　temperature.