Abstract: There exist two geomorphologic types (landscapes) on the area 40 meters away from the upper reaches of Wenyuhe River, i.e. floodplain and slope. The plots are divided into two areas: 0—20 meters and 20—40 meters away from the riparian. The floodplain is in the former area while the slope in the latter. On the basis of field survey and riparian forest community classification, the differences of forest community structure and diversity of the two landscapes were compared, serving for riparian width speculation. The results are as follows. With TWINSPAN and DCA, the 56 plots belong to 13 kinds of community, and the stand density, species richness and plant diversity of riparian forest are higher than that on the neighboring slope, but the differences of tree breast-area and evenness of the forests between the two landscapes are not significant, while the species similarity of the two landscapes is high, the common species is accounted to 75.13%. The dominant tree species are similar in the two landscapes, but Larix principis-rupprechtii grown on slope is more outstanding than that on floodplain. There are 34 plant species exist only on floodplain, most of which are wetland plants. 13 xerophytes species are merely exist on slope. Based on the situation mentioned above, it is deduced that the influence of flood on plant flora is limited to the area 20 meters away from riparian.