The stiffness characteristics of a pneumatic soft actuator with high expansion ratio composed of latex airbag,fiber cloth and other materials were studied.According to the structural characteristics of the soft actuator,the static output force model of the soft actuator was improved by combining theory with experiment,and the static stiffness model of the soft actuator was established.Combined with the gas multi-variate equation,the dynamic stiffness model of the soft actuator was established,and the dynamic stiffness was the sum of static stiffness and pressure stiffness.The static stiffness test platform of soft actuator was built and its static stiffness was tested.The results show that the static stiffness of the soft actuator decreases with the increase of the expansion height,with a good linear relationship and a small relative error,when the air supply pressure is 70 kPa,the static stiffness of the soft actuator can reach about 15 kN/m.The simulation analysis of the pneumatic stiffness shows that the pneumatic stiffness is mainly affected by the expansion height,the air supply pressure and the excitation displacement,when the expansion height of the soft driver is 30 mm,the air supply pressure is 10 kPa and the excitation displacement is 5 mm,the air pressure stiffness is only about 1/6 of the static stiffness.