In this paper, the global stability of periodic solutions and walking control of a semipassive biped robot with telescopic legs are studied. An inverted pendulum with variable length is used as the robot model, and the biped robot is powered by elongating and shortening the stance leg. The fixed point and its stability of the biped robot walking are discussed based on Poincaré map. With the assumption that the angle between two legs keeps constant, when the swing leg impacts with ground at heelstrike, the control law of telescopic length of the stance leg is designed by feedback of the angle between stance leg and vertical line. Then, the global stability of the fixed point is proved for the biped robot walking with telescopic legs. The simulation results show that the proposed feedback control law can make the biped robot with telescopic legs walk stably on the level ground, and the cycle gait is robust to the various disturbances of the actuator and the disturbances of the initial angular velocity of stance leg.