Abstract: A kind of semi-active engine mount (SEM) with switchable stiffness decoupler is designed and manufactured, and the dynamic characteristics are measured with MTS 831 servo-controlled hydraulic test rig. The linear Lumped Parameter (LP) model for SEM is proposed. The methods for the calculation of dynamic stiffness and loss angle with high amplitude in low frequency domain are presented using the linear LP model. The dynamic properties of SEM are estimated with LP model, and the predicted results have a good agreement with the experimental data, which validate the proposed LP model. Some applications of SEM for solving NVH (Noise, Vibration and Harshness) problems in powertrain mounting system are illustrated. The presented modeling method and conclusions are contributed to research and development of SEM and hydraulic engine mount (HEM).