Objective To investigate the change in the posterior Diff value (distance from the top point of the posterior surface of the cornea to the perfect spherical surface) after laser in situ keratomileusis (LASIK) for myopia, and its relationship to visual acuity. Methods Comeal topography was performed on 62 myopic patients (123 eyes) before LASIK surgery and 1 day, 1 week, and 1 month after surgery. Subjects were divided into groups according to the thickness of the residual stromal bed, the ratio of the thickness of the residual stromal bed to that of the preoperative cornea or preoperative degree of myopia. The posterior Diff value and its relationship to visual acuity was analyzed. The posterior Diff value was compared between groups and interviews postoperation. Results The posterior Diff value was higher after LASIK surgery than before surgery. The value increased as the thickness of the residual stromal bed decreased, and it increased with an increase in the preoperative degree of myopia. When the thickness of the residual stromal bed was more than 300 μm or the ratio of the thickness of the residual stromal bed to that of the preoperative cornea was above 55%, the posterior Diff value clearly decreased. The posterior Diff value was related to visual acuity to some extent. Conclusion The posterior surface protrudes forward is elevated after LASIK. A change in the posterior surface is one of the factors that affect visual acuity. The changes are related to the thickness of the residual stromal bed and preoperative degree of myopia. LASIK may be safer if the thickness of the residual stromal bed is more than 300 μm or the ratio of the thickness of the residual stromal bed to that of the preoperative cornea is above 55%.