Semi-active vibration systems expend a small amount of energy to change the system parameters such as damping and stiffness. The variable damping and stiffness semi-active systems with base excitations have demonstrated excellent performances in the resonant and high frequency regions. For a forced vibration, it plays an important role in describing a building system with wind loading or a vehicle suspension with an engine vibration force; and the system stiffness has a large influence on the frequency response in the low frequency region. However, forced vibration systems with variable stiffness control have not been received significant attention. To address this issue, the performance of a variable stiffness system with a force excitation is studied in theoretical calculations and experiments. The responses to sinusoidal and random inputs showed that the system with stiffness on-off control had advantages of both soft and stiff systems in the whole frequency.