Pressure wave propagation along a pipe attached with an active pressure-controlling system at an end was described by an unsteady state-changing motion of the pressure wave fronts, in order to study the interaction among successive waves caused by successive controlling spillovers with the consideration of liquid compressibility due to high head. A model for water hammer propagation was established by using the continuity, momentum, and energy equations. The influence of the wave interactions on the pressure suppression was analyzed. Testing verifications were conducted with a 2575m length and 776m head penstock in hydro-power station. The computed results are in good agrement with the tested ones.