When the power system stability problem is concerned, the traditional view is to suppose that the electromagnetic transient process is finished, and hence its propagation along the transmission line can be neglected, and only the electro-mechanical transient process should be considered. In this paper, an analytical method is proposed to study the electromagnetic transient process of transmission line in a simple system. The system state variation in different conditions are compared and it is proved that the traditional method fit only in some common conditions. In a few extreme conditions, the electromagnetic transient process of transmission line must be considered and the system state variations should also be studied.