A long wave infrared optical system was designed and developed by setting the system parameters based on the performance of a non-refrigeration focal-plane array(FPA) detector and by calculating the sizes and detection distances of an infrared weak target, and then the target detection for weak signals was realized successfully. This infrared optical system based a field of view in 6°×6° is implemented with the image space centrifugal path. Analysis and calculation results show that the optical system with two pieces of Ge-based diffraction optics has advantages of lower weight, higher energy transmission ability and good image quality. As compared with that of the three-piece spherical surface Ge-AMTIR1-Ge system, the weight of the system has lighted by 25% and the efficiency has improved by 7%. Moreover, the image quality has improved 50% than that of the traditional aspheric surface Ge-Ge method. These results show that the optical system has a wider application future in infrared weak target detection fields.