Under the influence of global climate change,Madoi County in the source regions of the Yellow River is facing serious ecological and environmental problems,such as desertification,wetland shrink,grassland degradation and so on. By means of field investigation,interpretation of remote-sensing images,analysis of stratigraphy and grain-size analysis,causes and processes of desertification in Madoi County are analyzed in this paper. The results show that widely distributed fossil aeolian sediments and alluvial deposits of the Yellow River and its tributaries are the material sources of the desertification in the source regions of the Yellow River. The grain size characteristics of modern mobile dunes and fossil dunes are very similar,showing obvious relationship of inheritance and transformation of them. The distribution of the modern mobile sand dunes has not exceeded the scope of the fossil aeolian deposits. Due to environment changes,fossil aeolian sediments are subsequently fixed,and dense turf forms on the surface to protect underlying loose sand. Mechanical destroy of the turf is a crucial step for modern desertification. In Madoi County,mechanical damage of turf layer is associated with permafrost degradation. Permafrost degradation is different in different landforms. On fixed /fossil dunes,subsidence pits would form on the dunes due to permafrost degradation. Tensile cracks and extending falls would form around the edge of the pits leading to expose of underlying loose sand. On the slopes,slump scarps would be created by permafrost degradation,leading to expose the underlying soil. In the flat ground such as alluvial plain,waterlogged depression would form on the surface for differential frost heave and thaw settlement. With lowering of ground water level due to permafrost degradation,the bottom layer of the soil would be exposed. The destruction of turf leads to exposure of the underlying aeolian sediments. The aeolian sediments are eroded by wind,creating lateral recession,resulting in the upper soil collapse and leading to more aeolian sediment exposed. Alternation of wind erosion and gravity erosion would expand the blowouts,slump scarps,wind erosion patches,leading to merger of them,and eventually formed mobile dunes,eroded badlands and gobi deserts.