The range-gated imaging (RGI) technique based on near-infrared or short-wave infrared pulsed laser illumination can reduce the influence of backscatter effectively, so it can improve the operating distance of the optical imaging system in night and adverse environment. It is researched and brought forwards the operating distance model and the forecasting method of the RGI system according to the needs of the design and analysis of near-infrared RGI system, that is according to the traditional continuous (passive) imaging model calculations or test the performance of RGI system, based on the minimum resolvable contrast (MRC), using an equivalent exposure time integration principle, imaging results of periodic pulsed laser are converted, and then the equivalent band patterns RGI system is used for predicting the role of different target distances. The forecasting results and the actual experimental results have good consistency which indicate that the method is effective. This method can be used to analyze the design and function in many kinds of near-infrared RGI system, and may also apply to forecast the operating distance in the short-wave infrared system.