The injection molding of Fresnel lenses requires precise control of processing conditions to maintain optical accuracy. Each lens contains concentric microgrooves that must be reproduced with minimal distortion to preserve light-focusing performance. In this study, numerical simulation and statistical design were combined to optimize molding conditions for a PMMA Fresnel lens. The Taguchi method and Grey Relational Analysis were used to evaluate five key parameters: melt temperature, mold temperature, injection pressure, packing time, and cooling time. Volumetric shrinkage and total displacement served as indicators of geometric accuracy, and the Grey Relational Grade was applied to assess overall performance. The optimal condition corresponded to a lower melt temperature and an extended cooling time, which reduced deformation and improved uniformity in the optical zone. Analysis of variance revealed that melt temperature had the most significant influence, whereas packing time showed a secondary effect. The results emphasize the importance of thermal control in achieving stable lens geometry and provide guidance for mold compensation in precision polymer optics.