Among　the　rare-earth　permanent　magnetic　materials,　Sm-Co-based　alloys　exhibit　superior　magnetic　properties　at　high　temperatures.　However,　their　high-temperature　applications　are　limited　by　their　relatively　low　saturation　magnetization　and　structural　stability.　The　stability　and　magnetic　properties　of　these　alloys　can　be　enhanced　by　adding　proper　alloying　elements.　In　the　search　for　new　permanent　high-performance　magnets,　researchers　have　systematically　investigated　the　structures　and　magnetic　properties　of　variously　doped　Sm-Co　phases.　The　present　research　investigates　the　structural　stability　and　magnetic　properties　of　Sm-Co　based　alloys　using　a　crystal　structure　model　of　SmCo7　with　an　accurate　atomic　ratio　(Sm:　Co　=　1:7).　To　explain　the　coexisting　multiphase　phenomenon　observed　in　experiments,　Mn-　and　In-doped　Sm-Co　alloys　were　modeled　by　first-principles　calculations.　Systematic　calculations　were　conducted　on　these　models,　and　were　combined　with　thermodynamics　calculations　to　determine　the　preferred　occupation　sites　of　the　doping　elements　and　their　change　rule　with　temperature.　Based　on　the　calculated　energy　and　electronic　structures,　the　structural　stabilities　of　the　alloys　were　studied.　The　Mn　and　In　dopants　influenced　the　interactions　among　the　Co　atoms　in　the　studied　alloys.　A　microscopic　mechanism　of　stability　improvement　in　the　SmCo7-based　alloys　was　then　proposed.　The　magnetic-moment　calculation　showed　that　the　Mn　additive　enhanced　the　total　magnetic　moment　of　the　SmCo7　alloys.　This　finding　explains　the　effects　of　doping　elements　on　the　saturation　magnetization　of　the　SmCo7　alloys.