Abstract: To address the critical challenges of cold reflection suppression, stray light control, and thermal stability in cooled mid-wave infrared (MWIR) optical systems, an innovative design based on a catadioptric secondary imaging structure was proposed. Through co-optimization of a modified Ritchey-Chrétien (R-C) reflective subsystem and refractive components, the system achieved strict focal length matching (total length was 166.67 mm) with an F-number of 2, a maximum aperture ≤85 mm, a field of view of 4.398° × 3.519°, and a 100% cold diaphragm efficiency. Utilizing an optical passive athermalization scheme with titanium-aluminum alloy thermal compensation, the system maintained stable modulation transfer function (MTF) >0.23 (@50 lp/mm) and RMS spot radii smaller than the Airy disk over a temperature range from −40 ℃ to 60 ℃. For stray light suppression, the joint optimization of YNI and I/Ibar parameters reduced the narcissus-induced temperature difference (NITD) from 5.17 K to 2.57 K. LightTools-based ghost image analysis confirmed that the secondary reflection paths contributed less than 0.01% irradiance ratio. Combined with an internal baffle design, the system stabilized point source transmittance (PST) below 10⁻⁴ at 4°~89° off-axis angles, achieving one-order-of-magnitude attenuation in the 4°~30° range. The proposed integrated opto-mechanical-thermal-stray light optimization framework provides a reusable engineering paradigm for compact, high-precision infrared systems with ultra-low background interference.