The three-dimensional tree model is widely used in the fields of virtual geographic environment, three-dimensional city scenes, etc. However, due to the rich geometric details of trees, it is still a huge challenge to effectively render trees in large-scale forest scenes. In this study, a real-time simplification method for tree foliage based on Viewpoint Mutual Information (VMI) is proposed. In the preprocess, the whole tree is divided into parent-child branch and leaf nodes according to its own topological relationship. Then, the average importance of each leaf under multiple viewpoints is calculated based on the VMI value and the leaves are sorted based on it. The leaves of less importance are pruned first during the real-time simplification process. In the real-time simplification process, we propose a view-dependent simplification method which greatly reduces the number of primitives needed to be rendered. In order to improve the rendering performance in forest scenes, a variety of rendering optimization measures are taken to avoid unnecessary Level Of Detail (LOD) transition.