In order to quantitatively analyze plant root growth and construct plant root growth model, a root system representation model based on tree-form data structure is proposed in this paper. After processing and analyzing root image, the information of root, such as length, diameter, number of child root, position of child root, angle of child root etc., is stored in a node of this treeform data structure. The accurate representation of root system is realized, which provides a foundation for the extraction and analysis of various root parameters. The specific algorithm is presented too.