Abstract: In order to ensure that the robot can work safely in human-s living environment, the most stable trajectory should be selected to finish the task. A new stability evaluating criterion which can be used to evaluate the stability of different walking patterns of the humanoid robot was proposed. The distance between the ZMP(zero moment point) and the center of the supporting region was defined as instability capacity, and the instability capacities of several typical walking patterns were analyzed. In addition, the motion planning method based on instability capacity was proposed. In some cases, for an appointed task, many kinds of motions which have different step length can be used to finish the task, and the most suitable trajectory can be selected among those feasible trajectories using the instability capacity evaluating method. The effectiveness of this method was proved by simulation results.