Abstract: A semi-active hydraulic damper includes a cylinder, an accumulator, an oil box, and a directional control member. A directional control valve of the directional control member is connected to a first, and a second primary oil circuits, which are respectively connected to the ports of two ends of the cylinder body. The valve can switch among various positions according to signals of a sensor provided for sensing movement of the structure. Vibratory movement of the structure that causes a displacement of the upper joint relative to the lower joint of the structure can be counteracted by pressure of oil, which is allowed to flow into the cylinder from the accumulator via selected one of the ports to move the piston in a direction opposite to the displacement by switching the directional control valve.