Abstract: The waist of humanoid robot plays an important role on the kinematics and dynamics characteristics of the whole body. This paper described problems of the humanoid robot waist and presented a flexible waist that could vary the joint stiffness. The influence of this waist on the compliant characteristics and the stability of the whole body were studied. This waist can stabilize the humanoid robot by performing the waist and the arms dynamic cooperative motion and realize security by varying the joint stiffness while operating.