The generalized highefficient recursive dynamics model of a flexible tree robots system with active-passive joint was developed in this paper, where the tree robots system was described by the topology method and the lower body array. Meanwhile, the method to describe the under-actuated rigid-flexible multibody system based on Spatial Operator Algebra (SOA) theory was proposed. The hinges were defined as active or passive joints according to the driving conditions of the joint in the system. The dynamics model was then constructed. Subsequently, the generalized articulated inertiamatrix, the residual forces and the generalized acceleration and torque were computed through twice tip-to-base recursive and once base-to-tip recursive. The generalized torque and the generalized acceleration was linear added at the branch. At last, the generalized dynamics model of the flexible tree robots system was generated. The highefficient O(N)th (N is degrees of freedom of robots system) computation efficiency was realized through the developed dynamics model in above recursive process. Simulation results show that the dynamic modeling and fast integration techniques proposed in this paper are correct and efficient.