Abstract: For the underactuated bipedal stable walking control, a feedforward control strategy based on the motion state of robot center-of-mass (CoM) is proposed. Firstly, an intuitive expression of the stability of underactuated walking based on the walking speed is proposed according to the relationship between walking speed and walking stability, and a mathematical definition is given as follows: if the robot walking velocity can always converge to a certain value which has been proved available for realizing at least one more cycle walking, then the robot walking is in a stable state. Secondly, inspired by the mathematical definition above and the gait characteristics of human variable-speed walking, a feedforward control strategy based on the motion state of CoM is proposed. The horizontal velocity of robot CoM is taken as system output, its control is achieved by controlling the displacement of CoM within a single walking cycle, and thus stable walking is realized. Finally, the underactuated bipedal walking with the average walking speed of 0.178 m/s and the step of 0.31 leg length is realized on the concrete and wood grounds respectively. The experiment results show that the underactuated stable walking can be realized with the proposed control strategy by controlling robot CoM to track the ideal velocity.