For the influence of environment effects on infrared search and track (IRST) system, the traditional operating range equation is changed in expression after analyses the parameters which relate to the environment. In the new operating range equation, the noise equivalent power (NEP) measures all noise power including environment noise. Based on the statistic characteristics of the signal and noise in digital image, the NEP can be calculated by the detector’s sensitivity. As a result, the operating rang of infrared system can be calculated in different applications. By analyzing the experiment, the conclusion is that comparing with traditional operating range equation, the results of improved operating range equation are more closer to the actual operating range.