The concept of postural synergy is recently being widely applied to the mechanical design of prosthesis. However, there are few research literatures on the application of postural synergy in the mechanical design of an exoskeletal rehabilitation robot. In this paper, the reaching movements of human arm are analysed by the principal component analysis method and two most significant synergies of the joints of human arm, which can account for more than 80% of the variation, are extracted. We propose a postural synergy based method to design the kinematic transmission mechanism for a multi-joint upper-limb exoskeletal rehabilitation robot with two actuators. Additionally, the configuration of passive joints and balance weights is put forward to improve the performance of the robot. The postural synergy based design method is formulated for replicating human arm reaching movements. Finally, tests are taken on the prototype to validate the proposed method. Furthermore, the proposed method can be potentially extended to the design of underactuated robotic arm or manipulator which is just required to perform the specific types of movements. (c) 2017 Elsevier B.V. All rights reserved.