It is of importance to monitoring defects of GIS basin-type insulators for guaranteeing the security operation of power systems. The vibration method becomes a research hot topic in fault diagnosis of electrical equipment due to its high sensitivity and nondestructive test. In order to provide theoretical basis for the vibration methods, three dimensional models of normal or defective insulators are established in this paper, and modal frequencies of insulators are calculated based on the finite element method under different chamber pressures and different crack sizes. It is shown that doublet modes occur when there are no cracks in basin-type insulators, but there are no doublet modes for insulators with cracks. When compared with the condition that the chamber pressures of both surfaces are the same, the modal frequencies decrease as the chamber pressures of lower surfaces decrease, and the modal frequencies increase as the chamber pressures of upper surfaces decrease. When there are cracks in basin-type insulators, the modal frequencies decrease, and with the increase of the depth of cracks, the decline ranges of modal frequencies increase. These simulation results will no doubt prove the feasibility of using the vibration method to monitor the mechanical condition of basin-type insulators.