CO　oxidation　on　phosphomolybdic　acid　(H3PMo12O40,　PMA)　supported　single-metal　atom　(M　=　Pt,　Au,　Co,　Cu,　Fe,　Ir,　Ni,　Os,　Pd,　Ag,　Rh,　and　Ru)　(M-PMA)　catalysts　is　studied　by　density-functional-theory　(DFT)　calculations.　Adsorption　of　CO　and　O-2　on　M-PMA　is　investigated.　Based　on　electronic　structure　analysis,　O-2　is　activated　by　the　single-metal-atom　active　center.　The　Langmuir-Hinshelwood　mechanism　is　systematically　explored　for　CO　oxidation　on　M-PMA,　and　it　is　found　that　M-PMAs　have　high　reactivity　toward　CO　oxidation.　The　Mars-van　Krevelen　mechanism　is　also　investigated　and　it　is　shown　to　be　less　likely　to　be　responsible.　Our　DFT　findings　will　provide　useful　insight　for　designing　stable,　highly　active　heteropolyacid-supported　single-metal-atom　catalysts.