Hybrid electric vehicles often experiences key-on/off conditions. Increasing vibration isolation performance of the powertrain mounting system (PMS) under key-on/off conditions is one of the important measures to enhance NVH (noise, vibration and harshness) performance of the vehicle. A model with 13 DOFs for PMS was established, and acceleration of the mounts at active side was estimated under the key-on/off condition. Also, vibration acceleration of the mounts at positive side, passive side and the seat rail were measured. The comparison between calculations and experiments validated the proposed model. Deterministic and uncertain optimization methods for reducing the vibration dose value of longitudinal acceleration of the powertrain were proposed, and the mount parameters were the design variable. Key on/off experiments were carried out to validate the proposed methods. The experimental results show that the two optimization methods can effectively enhance the vibration performance of the PMS, and the uncertain optimization method can make the PMS more robust and effectively deal with the influence of parameter uncertainty.