The　first　principle　calculations　are　employed　to　investigate　the　Ag-doped　Mg3Sb2.　Comparing　the　formation　energy　and　the　bonds　length　between　the　Ag　atom　and　its　nearest　Sb　atoms,　it　is　found　that　Ag　atom　is　more　favorable　to　substitute　Mg　atom　in　the　anionic　[Mg2Sb2](2?)　layers.　Comparing　the　electronic　energy　band　structures　between　the　un-doped　and　Ag-doped　Mg3Sb2,　the　Femi　level　across　the　valence　band　through　Ag　doping.　The　double　transport　mechanism　are　induced　by　the　doping,　at　low　temperature,　materials　exhibit　metallic　properties,　and　when　the　temperature　is　higher　than　422　K,　the　thermal　excitation　of　electrons　increases　gradually,　metallic　and　semiconductor　transport　mechanism　coexist.　As　the　temperature　continue　rising,　the　metallic　conduction　mechanism　returns　to　dominant.　The　composite　transport　mechanism　are　verified　in　our　calculations,　and　are　found　in　temperature　dependence　of　the　electronic　conduction　and　Seebeck　coefficient.　Our　calculations　explain　the　previous　experiments.