An optical system of a portable non-mydriatic fundus camera was designed. This design realized the highly clear imaging with three million pixels. Based on the physiological and optical characteristics of the human eye, the Gullstrand-Le Grand eye model was introduced to simulate the human eye with normal diopter. The Schematic eye model was used to examine the influence of the ametropia eye on imaging system. In the imaging system, the eye-piece objective lens were firstly used to converge the light emitted through the pupil reflected from the retina of the human eye, then it was imaged to the CCD receiver by the imaging objective lens. In case of the stray lights caused by the cornea, the illuminating system with an annular aperture and coaxial illumination was specially designed. The result shows that the field of view of this system is 30 degrees, the resolution is more than 120 lp/mm, the field curve value is less than 0.12 mm, the distortion value is only -1.2%, the chromatic aberration of all fields of view is within the Airy disk. It comes to a conclusion that this fundus camera has strong focusing abilities for accommodation, and adapts to different eyes from -10 D to +10 D. All the optical components of the optical systems are common spherical glasses, which are easy to manufacture and can reduce the production costs effectively.