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Metasurfaces have shown excellent ability in manipulating light fields. However, the intrinsic chromatic aberration impairs the quality of their broadband operation, which is a serious problem restricting the application of metadevices, especially metalenses. In this paper, a polarization-independent broadband achromatic metalens composed of engineered structures that have spatially varying gradient sizes is devised to focus the light over the mid-infrared region from 3 to 5 µm. The proposed dielectric metalens is constructed by an array of meta-units with a hollowed square pillar. By tuning its structural parameters, the optical response of the meta-unit turns out to be broadband achromatic, insensitive to the polarization state, and of high efficiency. Numerical simulation results show that the designed metalens can significantly suppress chromatic aberrations over a broad mid-infrared and hold a focusing efficiency of nearly 75%. Our work is beneficial to improving the mid-infrared broadband imaging quality and detection efficiency.