A power system model with reasonable external network is needed for online dynamic security assessment. Network reconstructing ensures the validity of dynamic security assessment done in the internal network， as well as realizing dynamic equivalence of external network and online integration of external and internal networks. In order to improve the equivalence precision， a practical dynamic equivalence method， which preserves some buffer network close to the boundary， is proposed in this paper. A method to paste the external equivalent network onto boundary is developed to keep the load flow in internal network unchanged after the pasting by using multiple Qθ load flow method and sensitivity method. An implementation in a provincial power system in Central China is described to demonstrate the feasibility and applicability of the method proposed. A set of software based on the method has been developed and integrated into an online early warning and security countermeasure system for real practice.