The stray light analysis of a diffractive/refractive long wavelength infrared (LWIR) staring imaging system is shown. In this LWIR system, there is one diffractive optical element(DOE) fabricated by the diamond turning technology. In this paper, three sources of the stray light- unwanted diffractive order light of the DOE, the multiple reflected light of the optical surface, and the reflected light from the inner side of the barrel- are analyzed with the help of LIGHTTOOLS software. From the simulation result of 6 twice-reflections, it is shown that when the source is moralized, which results in the irradiance of 100 W/mm2 at the image plane, the twice-reflection result in 0.01 W/mm2 at the image plane; the inner side of the barrel, which has 10％ reflection, result in 0.01 W/mm2 at the image plane. The analyzed result is approved by the image get from the LWIR system.