A model eye with aspherical cornea and nature lens are used to be calculated by raytracing method (ZEMAX program). The calculation shows that, spherical aberration is a key factor to influence the image quality of the human eye. To the emmetropia eyes, the negative spherical aberration of nature lens can compensate the plus spherical aberration of the cornea to reduce the whole spherical aberration of the human eye and keep a better vision. But the aged nature lens has little compensation with the spherical aberration. In order to reduce spherical aberration, the intraocular lens is needed to be aspherical. For a give Q value, the influence on the spherical aberration is proportional to the surface power. To minimize the spherical aberration of the whole eye, negative Q is needed in intraocular lens.