Metasurfaces have recently emerged to realize flat optical components with various functions and providing potential capabilities in manufacturing active devices. In this paper, a doublet lens for mid-infrared wavelength with tunable focal length has been proposed. The designed metalens consists of two dielectric metalenses with different focal lengths patterned on a phase-change materials Ge2Sb1Te4. The doublet lens is proved to have above 13% change in the focal length with the crystalline states shifting of the Ge2Sb1Te4. And the designed doublet lens can be operated in mid-infrared region from 2.41 mu m to 2.60 mu m. In addition, chromatic calibration can be achieved by adjusting the crystalline phases of the Ge2Sb1Te4. This compatible metalens promises potential for a wide range of applications in MIR region such as multi-spectrum imaging, optical coupling system and micrography.