A hydraulic power generation system driven by compression air produced by fluid is provided. The present invention is featured in that it employs more than one group of windmills which can be driven by wind power, or utilizes hydraulic power, such as a river flow, tides at estuaries or bays to drive more than one group of hydraulic turbines. The rotation power of the windmills or the hydraulic turbines then drives a transmission and subsequently drives an air compressor to produce compression air. The compression air is compressed with a specific pressure and collectively stored in an air compression tank. The compression air stored in the air compression tank is guided to a water filled hermetic tank. The hermetic tank is connected with a hermetic water tower having a certain height, in which the hermetic tanks and the hermetic water tower are connected with a communicating pipe by which a pressure can be conducted. According to the Pascal's principle, when suffering the pressure of the compression air, the water stored in the hermetic tank is transmitted to the hermetic water tower up to a certain height thus obtaining a potential energy. The water is collectively stored at the certain height to obtain a large amount. Such a large amount of water can be downwardly guided to drive a hydraulic turbine which is connected to a power generator for generating power. After releasing the potential energy, the water can be conducted back to the hermetic tank via another communicating pipe for recycle use.