Abstract: Unsteady flow is the general form of the ground water flow in rock mass. The basic principle of fluid mechanics is utilized to extend the law of steady flow through a single fissure, which leads to the law of unsteady flow through the single fissure because of the variable boundary condition. It is proved that the law of steady flow through the single fissure is the special form of the law of unsteady flow through the single fissure by means of the deduced equations.