This paper analyzes grain size characteristics based on samples of flood plain substance composition collected systematically from Longmen Sanmenxia section of the Yellow River. In terms of spatial changes, the longitudinal distribution of alongshore flood plain substances is characterized by coarseness at upstreams and fineness at downstreams. In terms of horizontal changes, grain size of flood plain substances changes from coarse at forward fringe to fine at backward fringe. Comparisons of grain size graduation of flood plain substances with that of river bed substances indicated that substances of flood plain facies are finer than that of river bed facies. They differ distinctively from each other. Comparisons of flood plain substances with suspended particles showed that sediments deposited on flood plains are mainly the portion of coarser particles in incoming suspended load from upstreams. The paper also studies the impact of flood plain width and substance composition on river bed geometric morphologies. It is found out that river bed width depth ratio is very closely related with flood plain width as well as percentage of silt and clay content in flood plain substances. The results of the above studies indicated that the first important condition accountable for Yellow River wandering is intense channel accumulation, and then a broad flood plain wider enough to satisfy the great amplitude in wandering. In case finer particles in flood plain substances is low in concentraction with weak anti scouring capability, the river channel will become even shallower and wider, so aggravating the extent of wandering.