A displacement controlling device comprises a shaft having a displacement region, a movable unit connected to the shaft and configured to move within the displacement region, a damper connected to one end of the shaft, and a restraint unit arranged between the movable unit and the damper, wherein the restraint unit is configured to be driven by the movable unit from a first position toward a second position; the damper outputs a resistance when the restraint unit is at the first position, and the damper stops outputting the resistance if the restraint unit is at the second position.