The　main　contact　system　of　an　ACB(Air　Circuit　Breaker)　is　modeled　for　calculating　its　electro-dynamic　stability　and　thermal　stability,　which　applies　the　contact　bridge　model　to　describe　the　microcosmic　contacting　between　movable　and　fixed　contacts,　the　electric　repulsion　force　and　temperature　rise　of　contacts　are　quantitatively　analyzed　and　its　short-time　withstand　current　is　evaluated.　In　electro-dynamic　stability　calculation,　3D　transient　electromagnetic　analysis　is　used　to　quantitatively　calculate　the　effect　of　eddy　current　on　the　electric　repulsion　force.　In　thermal　stability　calculation,　the　improved　contact　bridge　model　is　applied　to　obtain　the　simulative　results　of　transient　thermal　field.　Two　schemes,　i.e.　shifting　shaft　hole　position　and　increasing　contact　ending　pressure,　are　introduced　and　their　effect　on　short-time　withstand　current　is　researched　based　on　the　established　model.　An　optimal　scheme　is　given.