Large　ancient　landslides　are　widely　distributed　in　the　Jinsha　River　suture　zone　of　the　SE　Qinghai-Tibet　Plateau.　Understanding　how　these　landslides　were　triggered　and　moved　is　important　for　future　landslide　risk　management.　Here,　a　detailed　investigation　of　an　ancient　river-damming　landslide　in　this　zone　is　conducted.　Field　survey　suggests　that　the　deep-seated　landslide　developed　in　a　scarp　rock　slope　composed　of　Mesoproterozoic　gneiss　and　Paleozoic　hornblende　schist.　The　debris　volume　of　the　landslide　is　estimated　to　be　3.3　x　107　m3.　Evidence　of　a　landslide　damming　event,　including　dam　remnants　and　upstream　lacustrine　sediments,　has　been　well　preserved.　Optically　stimulated　luminescence　dating　shows　the　landslide　formed　at　about　5.4　ka　BP.　The　geological　setting,　frequent　seismic　activity,　and　characteristics　of　the　landslide　source　area　suggest　that　the　landslide　was　most　likely　triggered　by　a　strong　earthquake.　The　movement　process　of　the　landslide　under　seismic　shaking　is　simulated　using　the　discrete　element　method,　with　the　novelty　of　applying　a　viscoelastic　boundary　to　the　model　to　avoid　seismic　wave　reflection　at　the　slope　base.　The　simulation　results　show　that　the　progressive　failure　process　of　the　landslide　lasts　120　s,　and　could　be　divided　into　three　stages:　(1)　rupture　of　the　rock　mass　at　t　=　0-10　s,　(2)　formation　of　a　sliding　surface　at　t　=　10-20　s,　and　(3)　rapid　movement　of　the　landslide　body　with　a　maximum　movement　rate　of　34　m/s　and　blockage　of　the　river　at　t　=　20-120　s.　This　study　could　provide　guidance　for　understanding　the　dynamic　process　of　ancient　landslides　in　the　Qinghai-Tibet　Plateau.