There is great advantage when vibration is used to GIS (GIS: gas insulated switchgear) for condition monitoring and fault diagnosis. The paper researches the vibration mechanism when the GIS is normal from the aspects of electromagnetic force and magnetostriction. To explain the feasibility and necessity of vibration method, the paper classifies and analyzes the abnormal vibration. Natural frequency is the main factor that affects the vibration of GIS. The method of GIS modal analysis is researched in the paper. Different GIS air cell, model establishment and boundary conditions are analyzed. Natural frequency is calculated through modal analysis. The result is consistent with the modal test which certificates the correctness of modal analysis.