Infrared imaging is widely used in astronomical observation, medical diagnosis, and military applications. In recent years, metasurface technology has provided an unparalleled platform for the development of miniaturized and integrated infrared imaging systems. However, metasurfaces normally have inevitable chromatic aberration due to the high phase dispersion of the building blocks, which makes broadband achromatic infrared imaging difficult to realize. In this paper, we propose a polarization-insensitive metalens with a numerical aperture of 0.38 that can eliminate chromatic aberration for unpolarized incidences with the wavelength ranging from 3 to 5 µm. The simulated results show that within the design bandwidth, the proposed device achieves near-diffraction limit focusing and can increase the fill factor of infrared focal plane array pixels by 2.3 times, from 11.1% to 36.4%, with an excellent optical crosstalk performance of about 2.72%. Our work may pave the way for the practical application of achromatic metalenses in mid-wave infrared imaging equipment.