With the percentage of elderly people increasing every year, we proposed a walking assistance apparatus for disabled and elderly person, which is used for gait training and rehabilitation. We used zero-moment point (ZMP) control method to ensure stability of the walking assistance apparatus. Since users need to walk on stairs and the distance of every two stairs is invariable, the stride length of the apparatus is supposed to be constant. We compensated ZMP of the apparatus by adjusting the posture and the position of center of gravity (CoG). Thereby, the projection of gravity and inertia force can be maintained in the support polygon. From simulations and experiments, we can adjust the posture of the apparatus and user when instability occurs to prevent the user from stumbling. This method is suitable for walking on flat ground and stairs.