In order to study the electrical aging law of magnesium oxide insulating layer of magnesium oxide heating cable, electrical aging test of magnesium oxide was conducted. Based on the electric aging life formula - inverse power law, the life model parameters of magnesium oxide were estimated by constant voltage accelerated aging test, and then the reliability of life index n was evaluated by step-by-step voltage accelerated aging test. The results show that the life index n of magnesium oxide under constant voltage is 15.25, and the range of the life index of magnesium oxide under step-by-step boosting is 13.29–14.84. The life indexes measured by the two methods are basically the same, which verifies the reliability of the data of the constant voltage accelerated aging test and the accuracy of the fitted life model. Finally, the factors affecting the electric aging life of magnesium oxide were analysed in combination with the specimen pressing process and the breakdown theory of solid dielectrics.