The design of a modular wearable knee-ankle-foot-orthosis (KAFO) using novel soft actuator for post-stroke gait rehabilitation is presented. The configuration, different modules, working principles, actuation, control concepts, novel features etc. of the KAFO are introduced. As the actuation method plays the key role for the overall performances of the KAFO, the design, configuration, working principles, kinematics, dynamics, control analyses etc. of a novel soft actuation system are presented in details. The novel actuator is a variable impedance series elastic actuator designed with one motor and two types of springs in series, which is light in weight and compact in size. The actuator model is simulated for various conditions, and the results show satisfactory dynamic performances in terms of stability, safety, force bandwidths, variable impedance, compliance, efficiency etc. Then, the fabrication of the physical KAFO and its clinical validation with stroke patients are emphasized.