Abstract: A variable F-number infrared detector achieves dynamic matching with an optical system by adjusting the cold stop. To address the engineering challenges in practical implementation, this study investigated the thermo-mechanical environmental adaptability of such a detector. Based on an analysis of the structural characteristics and mechanical weaknesses of existing variable F-number detectors, thermo-mechanical coupled optimization was performed for the Dewar assembly and internal variable cold-stop structure, considering the thermal load constraints and mechanical reliability requirements. By optimizing the dimensions of the internal support rods, reducing the mass of the cold-end components, and improving the internal structural layout, the detector structure was further miniaturized and made lightweight while maintaining cooling performance and vacuum reliability. After optimization, the detector envelope size was reduced from φ85 mm×110 mm to φ60 mm×100 mm, and the mass was decreased from approximately 1650 to 950 g. Dynamic tests, high-temperature operation reliability tests, and imaging experiments verified the stability and reliability of the optimized thermo-mechanical structure.