To meet the requirements for a low-cost, compact, dual-field, and wide-temperature-range medium-wave infrared (MWIR) seeker, a dual-field MWIR optical system was designed based on a 640×512 pixel detector with a pixel size of 15 μm. The system adopts a four-lens imaging configuration using silicon/germanium (Si/Ge) materials. To achieve dual-field switching and athermalization over a wide temperature range, a single-lens axially moving zoom structure was employed, incorporating a refractive/diffractive hybrid optical element and an aspherical surface. Prototype results show that the system operates in the 3.7−4.8 μm spectral band, with an F-number of 3.97 and focal lengths of 65 mm and 21.5 mm, offering a 3× optical zoom. The corresponding field of view ranges from 8.4°×6.7° to 25.1°×20.2° at 33 lp/mm. The on-axis modulation transfer function (MTF) is no less than 0.3, while the off-axis MTF at 0.7 field is no less than 0.22. The cold stop efficiency reaches 100%. The system maintains good imaging performance across a wide temperature range (–40°C to +60°C) without significant Narcissus effects. The overall dimensions are no more than ϕ50 mm × 72 mm, and the total weight does not exceed 36 g. The system features a compact structure, ease of fabrication and alignment, high production yield, and strong feasibility for engineering implementation. Its performance metrics meet the requirements of practical applications.