To expand the design freedom of metalenses, realize multifunctional metalenses, and reduce the design difficulty of metasurface devices, an inverse method based on gradient-descent optimization is proposed. Combined with the backpropagation algorithm to solve the gradient information, the polarization-insensitive achromatic metalens design based on a single-layer structure is realized. Using two-photon polymerization three-dimensional (3D) printing technology, the processing technology was investigated, and a metalens with a lateral resolution of 200 nm was processed. Experimental results revealed that the focal length of the processed metalens deviated from the design value by less than 3%. Compared with other studies, this study combines the design flexibility of the inverse design method with the 3D processing advantages of the two-photon polymerization 3D printing method. As such, a new solution for designing and processing high-degree-of-freedom metasurfaces is provided by fully exploiting the advantages of light field control of metasurfaces, thereby promoting the engineering application of metasurfaces.