During　the　continuous　uplift　of　the　Qinghai-Tibet　Plateau,　the　genetic　mechanism　of　giant　ancient　landslides　in　the　deep-cutting　rivers　has　attracted　widespread　attention.　The　formation　of　giant　ancient　landslides　is　closely　related　to　the　evolution　history　of　rivers.　Using　the　methods　of　field　investigation　and　numerical　simulation,　the　formation　and　evolution　process　of　the　Zhenggang　giant　ancient　landslide　of　the　Lancang　river　upstream　is　studied.　The　correlation　between　the　landslide　formation　and　the　valley　evolution　is　analyzed.　The　toppling　deformation　body　evolution　of　a　giant　ancient　landslide　and　its　genetic　mechanism　is　studied.　The　results　show　that　the　bank　slope　unloading　caused　by　the　down-cutting　of　the　river　is　the　main　effect　factor　of　the　landslides　occurrence.　The　structure　of　soft　and　hard　inter-bedded,　and　rainfall　infiltration　are　all　important　influencing　factors.　The　bank　slope　unloading　caused　by　the　down-cutting　of　the　river　promoted　the　bending　and　toppling　of　the　steep　rock　mass.　The　giant　ancient　landslide　takes　the　toppling　fracture　zone　as　the　slip　zone　slipped.　©　The　Author(s)　2024.