Orifice-type viscous dampers exhibit a hysteresis behavior which is explained by additional stiffness. When mixed with air, its efficiency and additional stiffness would change. To study the dynamic characteristics of the damper mixed with air, calculated the nonlinear damping force and stiffness in a series model. A linear model in parallel which was convenient in practical application was established based on equivalent damping and stiffness, then researched on the efficiency and additional stiffness of dampers mixed with air in different frequencies and amplitudes. Experimental results of viscous dampers imply this method and model is correct.