Preparation　of　thermally　stable　metal　single-atom　catalysts　(SACs)　is　a　challenge　in　heterogeneous　catalysis,　especially　on　conventional　supports　that　provide　a　weak　metal-support　interaction.　In　this　work,　we　report　that　a　modified　support　MgAl2O4　can　stabilize　Pt　single　atoms　by　a　mechanism　of　vapor-phase　self-assembly　in　a　high-temperature　treatment　(800　degrees　C,　air).　The　experimental　results　on　the　formation　mechanism　and　the　structure　are　validated　by　DFT　and　ab　initio　molecular　dynamics　simulations.　We　infer　that　stable　triangular　K3O3　structures　help　stabilize　Pt　single　atoms　at　high　temperatures　in　oxidizing　conditions,　exhibiting　excellent　reactivity　for　methane　oxidation.　The　obtained　Pt/K/MgAl2O4　SAC　presents　excellent　stability　in　methane　oxidation　after　steam　treatment　at　elevated　temperatures,　whereas　the　Pt/MgAl2O4　nanocatalyst　suffers　from　rapid　deactivation　due　to　Pt　nanoparticle　growth.　This　work　paves　the　way　for　preparing　thermally　stable　and　highly　active　SACs　using　conventional　high-surface-area　supports,　despite　the　weak　metal-support　interaction.