The diffractive optical system has the advantages of large diameter and light weight, but its spectral range is narrow and the infrared signal energy that can be used is small. Therefore, it is generally believed that the detection signal to noise ratio (SNR) of the infrared camera will be reduced when it is adopted. Based on the diffractive optical system, the target detection performance of the ground observation infrared camera was analyzed, combined with an example of the calculation of the SNR of the uncooled infrared camera, the viewpoint that infrared cameras based on diffractive optical systems may still have good target detection performance was clarified when the spectral characteristics of ground object background and target were different. Meanwhile, the equivalent noise power of the infrared camera was compared with the laser and the electronic system. As a conclusion, the viewpoint that the performance of the infrared detection system may be further improved was proposed, and a method of introducing the laser local oscillator combined with the electronic filter which can subdivide infrared spectrum to reduce equivalent noise power was given.