A method for passive optical athermalization design of middle wavelength infrared optical system with multiple lenses is introducedand an athermalized middle wavelength infrared dualFOV optical system was designed.The optical system contains a zoom objective system and a secondary imaging systemand adopts the technology of passive athermalization optical design.The zoom function is achieved by moving a set of optical components (2 elements) axially.A passive athermalization optical design is presented by using crystal materials Si and Geand two aspherical surfaces.The results show that the infrared zoom optical system in the 4.5~5.1 μm wavelength possesses a compact structure100% cold shield efficiencyand better image quality in the temperature range from -60 ℃ to +70 ℃thereby the correctness and feasibility of the design method are proved.