Optical systems equipped with focusing ability are crucial for space exploration. The internal focusing optical system was designed which was based on the vertical deflection of the freeform surface. Specifically, the system used a special structure of the freeform surface to establish a lens group. It used the zoom ability of the vertical deflection to improve the imaging location thus adapting to different conditions such as wide range imaging or diverse space environments. The zoom theory of the freeform lens group was analyzed, and zoom lens of freeform was successfully applied to the optical system whose focal length was 100 mm. Then the imaging quality and displacements were analyzed when the object and image distance were changed accordingly. The results show that the Modulation Transfer Function (MTF) of the optical system is better than 0.3 at the Nyquist frequency which means satisfying quality of image. The system is stable in imaging and can provide compensation on the micron scale.