The strategy for choosing the reference frame of a flexible body was introduced firstly, which should obey the small strain terms. Furthermore, a group of normal modes and constraint modes were described under the body reference frame. Then, the multibody dynamical differential equation was deduced using the principle of Jourdain virtual power. In addition, we induced the equivalent nonlinear stiffness of the hydraulic cushion based on Comment Water Convervation Law and Bernoulli equation. Finally, the lifting and forging working were carried out by numerical examples based on Simpack.