Abstract: In order to fully utilize the clean, flexible and economic characteristics of distributed energy(DE), it is needed to efficiently manage its comprehensive and flexible regulation characteristics. However, directly embedding the virtual power plant(VPP) model into the main grid scheduling is prone to the risk of privacy leakage and great computational burden of different energy entities. Therefore, it is necessary to aggregate the multi-category flexibility resources of VPP to calculate their equivalent external characteristics. To this end, this article proposes a VPP flexibility characterization method based on vertex search, which aggregates various flexibility within the VPP into the output power of the VPP nodes, whose mathematical essence is the projection of a high-dimensional flexibility space to a low-dimensional space. Then, by introducing the extrapolation-based vertex search method, we achieve the approximation of the projection by maximizing the Euclidean distance between hyperplanes. Finally, based on the flexibility equivalence method, we propose an evaluation method for VPP flexibility resources to explain the main grid operation cost and optimal solution changes before and after the VPP access from a geometric perspective. The case study shows that the proposed flexibility aggregation model can effectively reduce the operating cost of the main grids, and the proposed VPP value assessment method based on external characteristic equivalence can effectively evaluate the value generated by various types of flexible equipment under small changes of flexibility ranges.