为了准确、高效地计算火箭尾焰的红外辐射特性，提出了一种确定火箭尾焰区域的方法。作为基础工作，分别使用FLUENT 和有限体积法(FVM)完成尾焰流场和红外辐射的计算。分析了可能用来区分尾焰和周围大气的变量，如温度和组分含量等，并选择CO的质量分数作为阈值变量。即，在选定阈值后，计算尾焰红外辐射时仅考虑CO质量分数大于阈值的区域，忽略小于阈值其区域对整体辐射的影响。分别研究了尾焰尺寸、计算时间和辐射强度随阈值的变化规律，结果表明，随着阈值的减小，尾焰尺寸和计算时间迅速单调增加，尾焰的红外辐射强度不断波动，且波动幅度逐渐变小，最终趋于平稳。另外，算例表明，选择CO质量分数为0.0005作为阈值可以确定一个比较合理的尾焰计算区域。“,”Aimed at calculating the infrared radiation of a rocket exhaust plume exactly and efficiently, a method for defining the domain of an exhaust plume was reported. As a basic work, the flow field was calculated by using the CFD software FLUENT, and the radiation was calculated by means of Finite Volume Method (FVM). Then, variables that might be able to be used to differentiate an exhaust plume from surrounding atmosphere such as temperature and species composition were studied and the mass fraction of CO was chosen. When calculating the infrared radiation of an exhaust plume, only the part where the CO mass fraction bigger than the threshold was taken into account, and the rest were neglected. The change law of size of calculation domain, calculation time and infrared radiation with thresholds were studied. The results show that, as the threshold decreases, the size of calculation domain and the calculation time increase monotonously and rapidly, while the infrared radiation changes a lot at first and becomes stable at last. Besides, it was indicated by simulation experiments that the CO mass fraction 0.000 5 as the threshold is acceptable to define the calculation domain of the exhaust plume.