The output force model of elongation type pneumatic compliance actuator has been improved and the mathematical model of static stiffness and dynamic stiffness has been developed for the variable stiffness control. The dynamic stiffness is the sum of static stiffness and pressure stiffness. The result based on the experiment and simulation has shown that the air supply pressure is a major factor affecting the static stiffness and dynamic stiffness of elongation type pneumatic compliance actuator. The static stiffness and dynamic stiffness increase with the increasing of supply pressure linearly.