A relationship between the focal length of the double-liquid lens and other parameters such as the applied voltage, the dielectric constant of the double-layer dielectric film and the film thickness etc. was analyzed based on the cylindrical double-liquid lens model with double-layer dielectric film. And the tantalum pentoxide film with a high relative dielectric constant as an inter-layer dielectric layer and a relatively low dielectric constant for the outer dielectric layer which is a waterproof layer, were used to reduce the driving voltage of double-liquid variable-focus lens. The effects of the thickness of two dielectric films and the thickness matching relationship between them were described on the variable-focus range and the driving voltage of the double-liquid lens. The optimum film parameters were obtained on the condition of minimizing the drive voltage of the lens at a certain variable-focus range. Simulation results show that double-liquid lens can be driven by relatively low voltage when the waterproof film thickness is much thinner than that of the high dielectric constant thin film. Within a certain variable-focus range, the driving voltage of the double-liquid variable-focus lens can drop to 10 volts. But double-liquid lens will not make efficient use of the high dielectric property of the tantalum pentoxide film, if the thickness of waterproof film is equivalent to or higher than that of the high dielectric constant thin film.