In this study, a transmission type long-wavelength infrared microscopic imaging optical system operating in wavelengths of 15-35 μm is designed to meet the demand for cooled staring focal plane array detector containing long-wavelength infrared 32×32 pixel elements and exhibiting an elemental size of 50 μm×50 μm. The designed system adopts disposable imaging and comprises a series of lenses. A cold diaphragm is positioned at exit pupil of the optical path. Symmetrical double gluing lens combination is introduced into the system to correct the aberration. Further, an optical passive compensation technique is employed to realize anti-thermal aberration at temperatures of -20 ℃ to 40 ℃. The simulation results show that the designed system exhibits a modulation transfer function (MTF) value of 0.369 and an encircled energy concentration of >80% at a characteristic frequency of 10 lp·mm ^-1 when its center wavelength, focal length, numerical aperture, effective magnification, and spatial resolution are 27 μm, 14 mm, 0.25, 10, and 0.1 mm, respectively. Furthermore, a clear distinguishable image can be obtained using the designed system. The designed system satisfies the requirement of short structure and high resolution of cold optical system.