Infrared focal plane arrays (IR FPAs) play an important role in various infrared imaging systems. In order to improve the working temperature, quantum efficiency and sensitivity of IR FPAs, microlens arrays are usually used as light condenser for IR FPAs. Currently, materials of microlens array are usually different from the material of the infrared detector, so additional process means are required during integration of the two parts, which is difficult and inefficient. Based on metasurface, a planar solid immersion microlens array can be directly fabricated on the substrate material of the infrared detector, so that monolithic integration of the two can be realized. Aiming at the application of antimonide class type II superlattice infrared detector, a solid immersion infrared metalens based on GaSb substrate this was designed. The designed metalens worked in the mid-wave infrared band and could be applied to all incident polarizations. The designed focal length is 100 μm for all metalens devices designed here. Theoretically, the highest focusing efficiency at the target wavelength reaches 70.7%, and the numerical aperture (NA) reaches 1.15. This design can promote microlens array to be flat, ultra-thin and lightweight, simplifying the integration process of microlens array and infrared focal plane array. Besides, this design is expected to improve the detection efficiency of infrared focal plane device and reduce manufacturing cost.