For conventional refractive lenses, chromatic aberration inevitably occurs due to the refractive index variation of the lens material with the incident wavelength, leading to axial aberrations and lower imaging system quality. Achromatic metalenses have demonstrated a great capability to solve this problem and been extensively investigated. However, the metalens achromatic method involves construction of a unit structure satisfying a phase distribution greater than 0-2π or phase compensation. Although this design method can obtain a good achromatic effect, finding a unit that satisfies a linear distribution during design is difficult. In this paper, we use subregion discrete wavelength modulation to achieve broadband achromatism. The total number of structural units in each region is optimized for different incident wavelengths, and the internal and external ring unit structures are also optimized. This achromatic metalens exhibits a large aperture and a high numerical aperture in the 4.2-4.7 µm mid-infrared band (NA = 0.83). Our research has strong potential and application prospects in ultracompact imaging and laser beam shaping.