The　highly　dispersed　＂single-atom　(SA)＂　catalysts　on　oxide　supports　significantly　alter　the　catalytic　reaction　activity　and　selectivity　and　meanwhile　save　the　catalyst　utilization.　However,　preparation　of　SA　catalysts　remains　a　challenge　up　to　now.　An　approach　effectively　evaluating　the　stability　and　activity　is　required.　Herein,　density　functional　theory　(DFT)-based　first-principles　calculations　were　performed　to　evaluate　the　stability　and　photocatalytic　hydrogen　evolution　reaction　(HER)　activity　of　noble　metal　(NM　=　Ag,　Au,　Pd,　and　Pt)　SAs　loaded　on　TiO2　support.　The　chemical　potential-based　thermodynamic　model　was　employed　to　estimate　the　stability　of　SAs;　the　capability　to　trap　photoelectrons　on　surface　and　free　energy　of　hydrogen　adsorption　were　used　to　estimate　the　photocatalytic　HER　activity　of　SAs.　Compared　to　the　(101)　surface,　the　(001)　surface　is　more　feasible　for　preparation　of　NM　SAs　due　to　the　＂soft＂　structural　character　caused　by　incompletely　saturated　surface　atoms.　The　stability　of　SAs　on　the　(001)　is　getting　better　with　the　loading　density　lowering　except　for　Au　SA.　After　deposition　of　NM　SAs　on　the　(001),　the　photoelectron　was　extracted　from　the　subsurface　to　the　surface　around　the　NM　sites,　facilitating　the　proton　adsorption　and　reduction　process.　The　calculated　free　energy　of　hydrogen　adsorption　shows　that　the　photocatalytic　HER　activity　of　NM　SAs　on　the　(001)　changes　moderately　with　the　loading　density　but　is　very　different　than　those　for　the　TiO2　clean　(001)　and　bulk　NM　(111).　Both　stability　and　activity　evaluations　dictate　that　Pd　SA　on　the　(001)　is　the　most　promising　candidate　catalyst　for　photocatalytic　HER