Tunable-focus liquid lens is considered as a kind of novel device based on biological vision system,which has many advantages such as little volume and weight,easily to be integrated and active zooming. A method of designing zoom optical system without moving lens is demonstrated according the theory of Gaussian optics,in which the liquid lenses are used as core elements. And a bifocal zoom endoscope system is presented,in which the curvature of the liquid lens can be controlled by air pressure or hydraulic pressure. A zooming factor of 1.5 is realized. And the acceptable imaging quality is gotten at both focal length of 1.8 mm and 2.7 mm of the system.