In order to meet the requirements of low background, low power consumption of low temperature optical system and high environmental adaptability of infrared detector refrigeration components, the design idea of the Dewar main body (window, window cap and enclosure) maintaining low temperature and flexible adiabatic connection with the cooling surface of the cryocooler expander is proposed. Aiming at the characteristics of the engineering application of the Dewar flexible shell for cryogenic optics, this paper takes a Dewar component of a long-wavelength 12.5 μm 2 000 element infrared detector for cryogenic optics as an example. This paper proposes a bellows as a flexible shell for adiabatic connection. The design of thermal insulation, mechanics and associated heat leakage of Dewar flexible bellows is highlighted. The thermal characteristics of bellows under different thermal load conditions are verified, and the minimum temperature gradient is 37.22 K, the adiabatic thermal resistance is 1142 K/W, and the error is 37%. In order to comprehensively evaluate the performance of the flexible shell structure, the thermal vacuum and qualification-level mechanical tests are carried out for a long-wavelength 12.5 μm 2 000 element detector flexible shell Dewar component for cryogenic optics. The test results show that when the low temperature window works at 200 K, the detector works at 60 K, the heat leakage of the Dewar is 544 mW. Compared with the normal temperature condition, the power consumption of the cryocooler is reduced by 53% when working in low temperature condition,, and the 4 g random mechanical test is passed, which verifies the low temperature optics. It is reasonable and feasible to use the Dewar flexible bellows shell model, which provides an important reference for the subsequent structural engineering application of the Dewar flexible shell for cryogenic optics.