Abstract: Due to time-variant or unsteady structure in large deployable mechanisms,the topological configurations and number of degrees of freedom are changable with motion. This unsteady dynamics is more complex than the dynamics of steady mechanical systems. In this paper,deployable mechanisms for space station are classified, their kinematic pairs are defined, constraint characters and equations, geometric and kinematic representations of orbit, attitude and deployment motion, and mathematical model of dynamics considering the flexibility and thermal deformation of structures are presented; the disturbances caused by joint gap, friction,or internal impact within limited distance, gravity and aerodynamic drag are discussed. Finally,some conclusions of experimental study are also discussed.