Over the past several years, there has been a substantial surge in the utilization of metasurfaces in micro-electronics and micro-optical devices. As one of their applications, metalenses have aroused significant research interest. In this paper, a new optimization method based on micro co-fluctuation model is proposed to optimize the initial metalens for better focusing performance by adjusting the structural dimensions of nanopillars. A segmenting iteration strategy is used to legitimately reduce the computation space. Meanwhile, an improved Harris Hawks Optimizer (IHHO) is proposed to obtain the optimal solutions, in which a new formula for escape energy is defined to promote the diversity of solution space. The optimization results indicate that the proposed optimization method effectively enhances the focusing capability of the initial metalens with a numerical aperture (NA) of about 0.7 and an operating wavelength of 10.6 μm. The maximum field intensity of the central hot spot and the absolute efficiency have increased by approximately 59.02% and 71.13%, respectively, and the full width at half maximum (FWHM) remains nearly the same value, which provides the potential for achieving high-contrast imaging.