The powertrain mounting system (PMS) plays an important role in improving the NVH (Noise, Vibration, Harshness) quality of the vehicle. In all running conditions of a vehicle, the displacements of the powertrain C.G. should be controlled in a prescribed range to avoid interference with other components in the vehicle. The conventional model of PMS is based on vibration theory, considering the rotation angles are small, ignoring the sequence of the rotations. However, the motion of PMS is in 3D space with 3 translational degrees of freedom and 3 rotational degrees of freedom, when the rotation angles are not small, the conventional model of PMS will cause errors. The errors are likely to make powertrain interfering with other components. This paper proposes a rigid body mechanics model of the powertrain mounting system. When the powertrain undergoes a large rotational motion, the rigid body mechanics model can provide more accurate calculation results. The quasi-static equation for solving displacements of the powertrain C.G. is presented, and the Newton-Raphson iterative algorithm is developed to obtain the displacements of the powertrain C.G.. The methods for estimating the displacements of the powertrain C.G. and the reaction forces of the mounts are also proposed. With the transverse powertrain of a vehicle as example, two models are presented for comparing: one is the vibration model, another one is the rigid body mechanics model, and the displacements and reaction forces on the mounts under typical and extreme load cases are calculated respectively. The results show that in extreme load cases, the rigid body mechanics model can be calculated more accurately than the vibration model. The displacements can provide a prescribed range to avoid interference with other components in the vehicle. The reaction forces can be used as input loads for the design and fatigue test of mounts.