Tandem strip mill is a complex system composed of a plurality of working frames and auxiliary equipments,each frame is a multi-variable,nonlinear dynamic system,and interacts with the adjacent one by the moving steel strip. The strip mill system can be seen as a flexible multi-body mechanical system composed of multiple rigid and deformable bodies such as:frames,hydraulic equipment,roller system,moving strip et al. During the rolling process,relative movement occurs between the roll system and stock which contacting with each other at the same time. With the comprehensive consideration of roll system‘s vertical movement and work roll’s rotation,also with the rigid movement and elastic deformation of moving strip,this paper realizes the displacement field’s description of rigid body and deformable body in flexible multi-body mechanical system through dynamic analysis,and then determines the law of strong nonlinear inertial coupling and stiffness coupling effect caused by the system structural elements’ rigid motion and elastic deformation occurred simultaneously. Finally,the flexible multi-body system of tandem strip mill‘s coupling vibration mechanism model was established which can be used to analyze the roller system’s dynamic response and working stability.