Dynamic equivalence has been proved to be one of the effective measures to improve analysis speed with good precision. A new dynamic equivalence method based on the electromechanical distance is presented. Different quantified indexes are defined and used to describe the ties between different parts in the power system. The mechanical and electrical distances between different units and the transient dynamic response curves subjected to different faults are synthesized to find information about the electromechanical distance in the power system. The inner squared distances are used in the aggregation about the original system. Then the whole system is partitioned into internal and external subsystems according to the inner squared distances. Equivalence is carried out with the external system and equivalent parameters for the simplified system are calculated simultaneously. Finally， the simulation results concerning the New England 39-bus system show that the algorithm proposed has high accuracy and better calculation efficiency and the influence of the external system on the internal system is well retained.