We　report　an　Ag-1　single-atom　catalyst　(Ag-1/MnO2),　which　was　synthesized　from　thermal　transformation　of　Ag　nanoparticles　(NPs)　and　surface　reconstruction　of　MnO2.　The　evolution　process　of　Ag　NPs　to　single　atoms　is　firstly　revealed　by　various　techniques,　including　in　situ　ETEM,　in　situ　XRD　and　DFT　calculations.　The　temperature-induced　surface　reconstruction　process　from　the　MnO2　(211)　to　(310)　lattice　plane　is　critical　to　firmly　confine　the　existing　surface　of　Ag　single　atoms;　that　is,　the　thermal　treatment　and　surface　reconstruction　of　MnO2　is　the　driving　force　for　the　formation　of　single　Ag　atoms.　The　as-obtained　Ag-1/MnO2　achieved　95.7　%　Faradic　efficiency　at　-0.85　V　vs.　RHE,　and　coupled　with　long-term　stability　for　electrochemical　CO2　reduction　reaction　(CO2RR).　DFT　calculations　indicated　single　Ag　sites　possessed　high　electronic　density　close　to　Fermi　Level　and　could　act　exclusively　as　the　active　sites　in　the　CO2RR.　As　a　result,　the　Ag-1/MnO2　catalyst　demonstrated　remarkable　performance　for　the　CO2RR,　far　surpassing　the　conventional　Ag　nanosized　catalyst　(Ag-NP/MnO2)　and　other　reported　Ag-based　catalysts.