Basing on the statistical model of eyes, using self-consistency single lens approximation and matrix optical theorem, we have calculated the backward shift of focus under the condition of different myopia degrees, and the quantity of corneal ablation for three different causes of myopia. The calculation shows that the maximum ablation is only 50um less than one tenth of the total corneal thickness although the myopia is 1000 degrees.We also consider using optical system to alternate illuminative distribution to achieve adjusting myopia, based on the gaussian distribution of laser beam.