In this paper, an active impedance control strategy for a knee-joint orthosis is proposed to assist individuals suffering from lower-limb muscular weaknesses during the swing phase of walking activities. The goal of the proposed strategy is to decrease the human effort required for ensuring a successful knee joint movement during walking without sacrificing the wearer's control priority. In this study, a gait-phase based desired knee-joint admittance model is designed by analyzing the kinematic and kinetic characteristics of the wearer's shank-foot segment during walking. Moreover, the mechanical impedance of the human/orthosis system is adapted to the desired one using an active impedance compensation. The control approach was implemented using a knee joint orthosis and tested with four healthy subjects. The EMG signals of the short head of the biceps femoris and the vastus medialis are used as metrics to evaluate the effectiveness of the proposed strategy. The experimental results show that the human effort can be significantly decreased when the wearers are assisted using the proposed approach.