In order to research the walking instability caused by the gap collision of humanoid robot, a kind of threedegree of freedom (3-DOF) dynamic model with gap collision is established. As compared with the existing model,it could be better simulated the walking process during gap collision between foot and ground. Based on the dynamic model, the mathematical model was deduced and the dynamic response of the collision system under periodic excitation was calculated by modal superposition. The Poincare section was used as the closure interface. The chaotic dynamic analysis of the collision system is carried out, which chaotic movement characteristics are found.This research lays a theoretic foundation for the research and control of the collision system of walking space of humanoid robot.