Abstract: A method of controlling a hydraulic system, the hydraulic system including a ram cylinder unit having a cylinder and a ram, and a hydraulic pump and a reservoir used to supply hydraulic fluid to the cylinder, and hydraulically driving the ram using the hydraulic fluid so as to move against a specific load, the method includes determining what a present state is one of an initial state, a proportional steady state, and a later state, controlling the pumping rate, which is obtained by adding the flow rate corresponding to the volume loss due to the compression of the hydraulic fluid thereto, to control the ram in the initial state, and controlling the pumping rate, which is obtained by subtracting the flow rate corresponding to volume recovery of the hydraulic fluid due to the relief of compression of the hydraulic fluid therefrom, to control the ram in the later state.