Landslide　dams　are　formed　mostly　in　V-shaped　valleys,　with　poor　stability　and　a　high　risk　of　failure.　Once　landslide　dams　fail,　they　may　cause　floods　or　debris　flows　that　endanger　lives,　property,　and　infrastructure　downstream.　Therefore,　it　is　important　to　evaluate　the　stability　immediately　once　a　landslide　dam　is　formed.　The　geometry　of　a　landslide　dam　is　the　most　critical　to　its　stability　assessment.　However,　the　complex　terrain　and　geomorphological　conditions　in　the　disaster　area　often　make　field　surveys　difficult.　Aerial　photography　and　satellite　imagery　technology　are　usually　limited　by　poor　weather　conditions,　which　bring　severe　restrictions　to　the　timely　acquisition　of　the　geometry　of　a　landslide　dam.　In　this　paper,　a　discrete　element　simulation　model　was　proposed　and　verified　by　two　experiments,　and　then　a　series　of　numerical　simulations　were　performed　to　investigate　the　geometry　and　formation　process　of　landslide　dams　under　different　topographic　factors　including　the　motion　path　dip-angle,　the　river　bed　inclination,　and　the　angle　between　valley　slopes　(valley　angle).　The　results　show　that　the　geometry　and　formation　process　of　landslide　dams　are　significantly　affected　by　the　topographic　factors.　According　to　the　results　of　comprehensive　numerical　analysis,　a　simple　predictive　model　for　landslide　dam　geometry　in　V-shaped　valleys　was　established,　and　two　landslide　dams　with　data　collected　from　early　events　were　employed　to　validate　the　predictive　model.　Based　on　the　easily　accessible　topographical　factors,　the　proposed　model　accomplished　the　fast　prediction　of　landslide　dam　geometry.　This　research　provides　a　vital　basis　for　the　stability　evaluation　of　landslide　dams　and　can　be　used　for　the　disaster　emergency　response　before　the　dams　break.　©　2020,　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.