The uncooled thermal infrared imaging spectrometer has advantages of small size, low cost and so on, can monitor high temperature events in the extreme thermal environment. However，the difficulties in detecting objects with low signal energy, serious thermal infrared background radiation and other problems limit its development and application. Restraining self-radiation stray light is the key to successfully overcome these difficulties. A long-wave infrared imaging spectrometer is designed in this paper, with 7 to 14μm wavelength range and F number of 2.7. By establishing its optomechamical model, two-step method of suppressing self-radiation are studied. The first step is using a mechanical structure with a gold film. Its reflectance is more than 98% in the thermal infrared band, thereby lowering the emissivity of the lens surface. Compared to the blackened mechanical parts, the polished one can reduce the self-radiation by one order of magnitude. However, it is still two orders of magnitude higher than the irradiance of the 500K target, and the spectral image signal is submerged. The next measure is to add a shutter device at the spectrometer entrance slit, so that signals reaching on the image plane when the shutter is opened and closed can be detected consequently. The effective spectral image signal is extracted by the background noise removal algorithm. The measures effectively solve the problem that the self-radiation stray light affects the imaging quality of the uncooled thermal infrared imaging spectrometer.