The　failure　of　solder　joints　under　thermal　cycling　is　mainly　as　a　result　of　progressive　damage　process　of　solder　materials.　The　objective　of　this　study　was　to　investigate　the　damage　behavior　of　SnAgCu　solder　under　thermal　cycling　condition.　A　damage　model　was　proposed　and　employed　to　simulate　the　thermal　cycling　behavior　of　SnAgCu　solder.　The　proposed　damage　evolution　law　was　based　on　continuum　damage　mechanics　and　an　interaction　between　creep　and　fatigue.　Thermo-mechanical　cycling　and　thermal　cycling　tests　were　conducted　for　model　parameter　determination.　A　special　bimetallic　load　frame　with　single　joint-shear　solder　sample　was　designed　and　used　to　study　the　damage　evolution　behavior　of　SnAgCu　solder.　The　damage　variable　D=1-R-0/R　was　selected　and　measured　for　the　single　joint-shear　solder　sample　every　dozens　of　cycles　during　thermal　cycling　tests　to　verify　the　model.　The　damage　evolution　law　was　deduced　as　power　function　with　thermal　cycles　and　the　results　show　that　the　experimental　damage　data　can　be　fitted　reasonably　well　by　the　relationship　as　the　damage　model　proposed.　The　microstructure　evolution　of　SnAgCu　solder　under　thermal　cycling　was　observed　by　Scanning　Electron　Microscopy　and　the　damage　mechanism　was　analyzed.