Optical metasurfaces, ultrathin 2D artificial materials with subwavelength structures, offer exceptional electromagnetic manipulation capabilities, making them promising for photonic integration. Traditional design methods rely on the local periodic approximation (LPA), treating unit cells as infinite periodic arrays to reduce computational cost. However, this approach neglects near-field coupling between adjacent units, resulting in focal shifts of 9%–22% in metalenses. We propose an adjoint method-based optimization framework that accounts for inter-unit coupling via full-wave simulations, creating a parametric gradient optimization space to refine metalens designs. Experimental validation through simulations demonstrates that the optimized metalens achieves a focal shift of 0.1%–1.8%, significantly outperforming LPA-based designs while showcasing scalable optimization potential. This framework offers a novel, efficient approach for metasurface optimization, with applications in optical displays, augmented reality, and 3D imaging.