In recent years, more and more distributed flexible resources have been integrated in the smart grid, such as electric vehicles and renewable energy, which promotes the rapid transformation from the passive distribution networks to active distribution networks. However, this process also brings great challenges to the operation and optimization of the power system. This paper investigates the energy management framework in the active distribution networks of multiple distributed energy resource clusters. The framework aims to minimize the total cost, and also optimizes the power loss in the process of peer-to-peer energy transactions. In order to ensure the reliable and safe operation of distribution networks, relevant network constraints are incorporated. The proposed framework adopts Nash bargaining theory, which can obtain a mutually beneficial and fair solution, and encourage clusters to participate in energy transactions. Finally, several case studies applied to an improved IEEE 33-bus distribution network show that the proposed framework improves the flexibility, economy, and security of energy transactions, and has effectiveness and superiority to some extent.