Abstract: The ultra-thin and high-pixel mobile phone lens is demanded with the development of mobile phone market. Combining the non-spherical principles, an ultra-thin eight-mega-pixel lens system was designed by using optical software CODEV. Four plastic aspheric lenses, an IR cut filter and a sensor cover glass compose the system. From the object side to the image side, the first and the third lenses are positive lenses, the second and the fourth lenses are negative lenses, and aperture stop is located in front of the first lens. The F-number is 2.4, FOV angle is 65.5, the effective focal length and the back focal length are 3.731 5 mm and 0.31 mm, and the length of the total lens group of the optical system is 4.6mm. The MTF values of the most of FOV are more than 0.5 at 1/2 Nyquist sampling frequency, and distortion is less than 2%.