Phase change materials have good photothermal stability and are rewritable. As their optical properties change significantly after phase change, and the crystallization states of phase change materials can be precisely controlled by applying laser excitation with different parameters. Therefore, on the basis of different optical parameters of phase change materials in different crystallization states, a thin-film flat lens with a multi-order refractive index design is proposed. The typical phase change material Ge₂Sb₂Te₅ is used as the optical modulation medium, and the crystallization state of the dielectric film is controlled discretely in multi-order and multi-region according to the phase constraints required by the focusing lens. Flat lenses with different numerical apertures are designed, and the focusing parameters of lenses are simulated by the finite-difference time-domain method and Zemax separately to verify the imaging performance of the lenses.