Abstract: Background The floodplain which is an important part of the river ecosystem, can protect the biodiversity, decrease the over-exploitation of sources,keep the homeostasis of the ecological system, and improve the abilities of the self-adjustment and self-healing of the system for the improvement of the human living environment, then has an important significance for the river water and soil conservation and the river health protection. In order to prove the classification method of medium or small river floodplains in hilly areas, this paper takes the geometric feature parameters of the Lingshan River floodplains in Longyou County of Zhejiang Province as research objects. Methods The geometric feature parameters of the Lingshan River floodplains were obtained through ImageJ software. And based on the basic morphology indexes, including circularity ratio, compact ration, elongation, form ratio, and mean curvature, the principal component analysis was used to get two principal components, P1 and P2, the two principal components can represent the information of all the basic morphology indexes. Using the two principal components as the plane coordinate axes and according to their values, the quantitative analysis of floodplain geometrical morphology was carried out. Results The research result indicated that the floodplains were classified into four types: the short wide regular floodplains, the narrow long regular floodplains, the narrow long irregular floodplains, and the short wide irregular floodplains. Analyzing the area and amount of floodplains in 2003, 2010 and 2013, it could be concluded that the area of floodplains decreased by about 17.82% in Lingshan River from 2003 to 2013, and the area of short wide irregular floodplains declined fastest by about 60.76%. The geometrical morphology of floodplain obviously presented the narrow long and irregular trend. Then, taking the Muchen village floodplain and Mei village floodplain as examples, the Muchen village floodplain was short wide and regular in 2003. However, in 2010 and 2013, it became a short wide irregular floodplain. This is mainly because that the Muchen village floodplain is located in the lower reaches of Muchen reservoir. The dam enhanced the turbulent fluctuation of the stream of the floodplain fringe and the irregularity of the floodplain fringe. The whole Mei village floodplain in 2003 was separated from some unrelatedly small floodplains in 2010 and 2013, and the whole area reduced by about 60%. Since 2003, due to the effects of human activities and excessive utilization, the Mei village floodplain fringe has been becoming more and more irregular. Conclusions Therefore, the classification method proposed in this paper can reflect the length and width rate and the degree of edge rules of the floodplains. In addition, it can be applied to analyze the morphology change tendency of floodplains in medium or small rivers.