A new design method for a variable-focus optical system without mechanical movements is proposed. It is quite different from that for traditional variable-focus optical systems, and changes the focal length of the system by regulating the relative position of the lenses with a motor. An extended Young's equation suited for double-liquid variable-focus lenses is derived by means of Helmholtz free energy minimization method. The traditional Young's equation based on vapor-liquid-solid tri-phase systems is extended to liquid-liquid-solid systems. Based on the cylindrical double-liquid variable-focus lenses, a new-type variable-focus optical system is presented. The conditions under which the optical system with appropriate applied voltages cannot only change the focal length but also keep the image plane stable simultaneously are researched. The corresponding simulation results show that the zoom ratio of the system can reach about 1∶1.5.