Electromagnetic transient simulation of an actual large-scale power system usually takes long time. How to achieve the equivalent circuit for the external system and reduce the simulation system scale has all long been one of the key topics in the field of electromagnetic transient simulation. So far the main external system equivalent methods in this field are classified， and the main content and process of each method and its advantages and disadvantages are described and analyzed. Finally， a look-ahead for the development tendency and the key issues of the external system equivalent methods to be broken through for power system electromagnetic transient simulation are presented.