The article discusses the structure and model of a robotic system for the rehabilitation of the lower limbs based on a passive orthosis in the form of a serial RRRR mechanism and an active parallel 3-PRRR mechanism. Effective numerical methods and algorithms were developed and tested that made it possible to determine the minimum geometric parameters of the active parallel mechanism that ensure the movement of the passive orthosis within the workspace under clinical data when simulating walking. The structure is proposed. The basis parameters for the rehabilitation system design are investigated. To implement the developed methods, an effective algorithm, software package, and visualization system for exported three-dimensional workspaces in STL format were synthesized. The results of mathematical modeling and analysis of the results are given. © 2021, CISM International Centre for Mechanical Sciences.