Abstract: According to the grain sizes of soils and experimental data of the longitudinal depth sections gathered from the lower Yellow River valley,the paper deals with different soil type areas,different landform units,soil size distribution,distribution characteristics of elements in various grain sizes of soils and contents and longitudinal variations of elements at the longitudinal soil sections. The significance of soil grain sizes in ecological geochemical investigation has also been discussed. The grain size range that can represent the main material composition of soils in this region has been ascertained. Finally the paper proposes the best plan for sample sieving and processing in the ecological geological investigation of the region and the effective depth and horizon for collecting soil samples in the surface and deep layers.