Tibetan　Plateau,　whose　average　elevation　is　more　than　4500　m,　is　joined　the　basin　or　plain　whose　elevation　is　less　than　2000　m　through　the　steep　slope　relief　of　the　edge　of　the　Tibetan　Plateau.　When,　how　does　the　steep　slope　relief　formed?　Nine　Apatite　fission　track　ages　and　length　of　the　steep　slope　relief　from　the　main　thrust　fault　hanging　wall　in　the　middle　of　the　northern　margin　of　west　Kunlun　Mountains　are　analyzed　in　this　paper,　showed　that　fission　track　ages　from　the　steep　slope　relief　area　of　elevation　3900~4635　m　are　ranging　from　6.2±1.4　Ma　~0.9±0.3　Ma,　fission　track　ages　show　as　＂up　new　under　old＂　anti-order　distribution.　Thermal　history　modeling　shows　multi-stage　uplifting　and　exhumation　of　~5　Ma,　~3~2　Ma,　~2~1　Ma　and　since　~1　Ma.　Combined　with　previous　studies　results　and　geological　observations,　think　that　steep　slop　relief　what　we　now　see　in　the　northwestern　margin　of　Tibetan　Plateau　is　overstep　propagation　thrust　results　of　west　Kunlun　thrust　fault　system　since　late　Miocene,　it　experienced　the　multi-stage　overstep　propagation　thrusting　movement　of　~8　Ma,　~5Ma,　~3~2　Ma,　~2~1　Ma　and　since~1　Ma.　It　provides　an　important　chronology　constraints　for　age　and　causes　of　the　steep　slope　relief　forming　in　the　margin　of　the　Tibetan　Plateau　and　uplifting　of　age　and　type　of　Tibetan　Plateau.