Abstract: Since the present passive walking robots can only achieve a single gait and tend to tumble easily,the stable walking control problem for a semi-passive biped robot is studied.A quasi open-loop walking control method is presented through combining the advantages of passive walking and active walking.By detecting the signal values of contact sensors installed under the foot and at the kneecap,an intermittent and small open-loop oscillatory torque is imposed on the hip joint to realize stable walking with high efficiency.The simulation results show that the biped robot can achieve stable walking within a relative large region of control parameters and the energy cost of walking is similar to human beings.By modifying the parameters of the oscillatory torque,the robot can achieve stable transition between walking patterns.