As O-ring is widely used in GIS, research of its aging and operating life is helpful to the maintenance of GIS. In order to study the aging of O-ring and its compression set after aging, a method of aging experiment was designed. The experiment was carried out according to the method, and the results at different temperatures were collected. A data processing method is also introduced, and the results of the experiment were processed using according to the method to predict the operating life of O-ring at different temperatures. According to the results of the experimental study, the compression set of O-ring used in GIS is positively correlated with the aging time and temperature, and the aging process follows the Arrhenius rule; the operating life of O-ring is 2.5 years at working temperature of 70°C. The methods and conclusions of this paper can be used to evaluate the operating life of O-ring used in GIS.