Compared with the traditional keratometer which adopts double image prism to measure the corneal refraction, a novel keratometer′s optical system, based on cursor imaging principle with impact structure and convenience in use, is proposed in this paper. The optical system includes imaging system and illumination system. Imaging system, which consists of projection objective, cornea and photographic objective, adopts exterior focusing model. Illumination system adopts Kohler illumination, which realizes uniform illumination on the ring cursor, anterior corneal surface and the image plane on the CCD receptor. The image resolution can achieve 133 lp/mm, and the distortion is less than 1%. The measurement accuracy of corneal refraction reaches to 0.25 D (0.02 mm for curve radius), and the measurement range is 30~60 D (5.5~11 mm for curve radius). With high measurement accuracy and a wide range of measurement for the corneal refraction, the optical system simplifies the keratometer′s structure and operation for practitioners effectively.