A semi-active mount structure may include a nozzle plate mounted between an insulator and a diaphragm, to divide an interior of a main case into an upper fluid chamber and a lower fluid chamber, the nozzle plate being formed with first and second fluid passages, a plunger mounted in a housing coupled to a lower surface of the diaphragm such that the plunger is vertically movable, the plunger opening or closing the second fluid passage in accordance with application of electric power to a coil, a valve spring disposed between the plunger and the housing, to apply elastic force to the plunger, the valve spring dividing a space defined between the diaphragm and the coil into an upper space and a lower space, and a valve mounted at a vent hole drilled at one side of the housing for air to be introduced into or discharged from the lower space.