The virtual power plant（VPP）holds great potential in integrating distributed energy resources（DER）, which provides an efficient solution for the flexible management and effective utilization of grid resources. A flexible resource aggregation operation boundary estimation method for VPP is proposed to facilitate VPP participation in system dispatch. First, the active and reactive power operation constraints of DER are considered and expressed in a unified linear inequality form. Then, all heterogeneous DERs on the same distribution network node are aggregated into one DER aggregator by a geometric computation method. Finally, considering the network constraints, a VPP operation boundary estimation model is established, and the operation boundary is calculated using the boundary point search method. The case study results show that the proposed method can effectively estimate the multi-time operation boundary of VPP by considering the power coupling and time coupling characteristics.