Abstract: Deep-seated toppling deformation of steep stratified rock mass is a commonly-seen geological phenomenon in deeply incised valleys in Southwest China. Primarily contributing to this is severe valley downcutting in the wake of uplift of Qinghai-Tibet Plateau since the Quaternary period. In the present paper, taking the case study on the Yagong toppling fracture in the upper reaches of Lancang River as an example, bottom-friction gravity test and numerical calculation were employed to probe into the mechanism and process of how such toppling fracture has formed during valley downcutting. The study has drawn conclusions mainly as follows: the side slope moved along with the valley while downcutting; rock mass loaded off and the field of stress of side slope was re-distributed; a deep-seated area of stress relaxation took shape in the rock mass; and the direction of primary stress re-aligned with gravity direction, under the effect of which deep-seated stratified rock mass in the area of stress relaxation was made topple and deformed towards the valley. The study, on the basis of physical testing features and numerical analysis results, finally concludes that the evolution of such toppling and deformation has the stage of unloading-rebound tension fracture, the stage of deformation-crack development, and the stage of fracture-crack penetration.