In this paper, a transmissive metalens operating in the mid-infrared band with polarization insensitivity is designed. Using silicon dioxide as the substrate and silicon as the dielectric material, the metalens has a radius of 100 μm, an optimal focal length of 302 μm, a focal efficiency of 70.72 %, and a simulated focal length with only 0.0013 % error from the theoretical calculation. Taking the minimum value of the focal length error as a reference, several metalens with different radii were designed, and the data of optimal focal length, numerical aperture, and focusing efficiency are obtained, which improved the efficiency of the subsequent design of metalens based on the same structure. The design method provides ideas for calculating the optimal focal length of metalens, and offers technical support for improving the efficiency of the mid-infrared superstructured lens.