The debris flow in southeast Tibet area can divided into three genetic types: a rainwater type, a ice-snow melt-water type and a glacial-lake burst type. Theory of minimum energy dissipation is observed in longitudinal profile evolution of the debris flows. The theory is increase of velocity with adjustment of longitudinal gradient in valley. It is expression of the theory that average value (ū) of velocity along the valley relates to a forming index (N) of the valley longitudinal profile: ū∞f(N). The f(N) is a velocity function. For debris flow valleys of rainwater, melt-water and burst types, the f(N) are respectively {1/3-2/[(N+1)(N+2)(N+3)]}^1/2,{2/3-2/[(N+1)(N+3)]}^1/2 and [N/(N+1)]^2/3. In process of evolution of the valley, increase of the value N with increase of the value ū,and shape of the valley longitudinal profile develop to a concave parabola pattern from a convex one, and morphological stages of the debris flow are a brow stage, a developing stage, a exuberant stage and declined stage, and stable stage of the basin in order. As examples with model debris flow valleys in southeast Tibet area, above model and regular of morphological evolution in the valley are examined.