To aim at the characteristics of combination of pneumatic elastomer of flexible joints, the uniform distributed mass of pneumatic artificial muscle (PAM) was equivalent to the concentrated mass of the two ends by means of equivalent mass method; then the kinematics differential equation of the PAM was set up; and the natural frequency of PAM was consequently obtained. Based upon the above analysis, the dynamic model of joint with variable parameters was then analyzed; both the axial and bending dynamic performance of the two directional active bending flexible joint was studied and their natural frequency separation was obtained. And this study provides basis for making humanoid robot hand using this flexible joints.