The Kane's dynamical model of flexible multibody space systems with tree structure is developed in this paper. The system topology is restricted to a tree configuration，which is defined as an arbitrary set of flexible and rigid bodies connected by hinges characterizing relative translations and rotations of two adjoining bodies. The relative translatioal velocities，angular velocities，and the differential of model coordinates are selected as the generalized velocities. The motion equations of minimum dimension are derived via Kane's method. The resulting equations are suitable for automatic generation，and simulated by computer.