To solve the problem of constraint between resolution and depth of field in capsule endoscope system, a single-liquid, electron-controlled, focus-tunable len was applied into the design of capsule endoscope system. By using of liquid lens, the depth of field of system was enlarged with the required resolution. The initial system was set up by the optical design software CODE V. The liquid lens model was built according to its structure restriction. The liquid volume of the lens was constant,while the radius, the aperture and the thickness of the lens were changed during the focal length changing process. At different depth of field, the appropriate radius,aperture and thickness of the liquid lens were calculated by MATLAB, and the calculation result was imported into CODE V. The optimizing process was designed. After the optimization, in 3~100 mm depth of field, clear images can be obtained. The field of view is more than 110°, and the modulation transfer function of the system is larger than 0.3 in all field of view at 40 lp/mm.