The operation range is one of the important parameters for describing the whole performance of a military infrared imaging system. It can be divided into detection range, recognition range and identification range among which the detection range is the maximum one. According to the geometric relationships among the infrared imaging system, the earth surface and the target, a method for calculating the operation range of an earth observation infrared imaging system based on MATLAB is presented. The value of the operation range is determined by the altitude of the infrared imaging system, the radius of the earth and the height of the target. The actual detection range of an infrared imaging system or the operation range proposed for a system to be developed should not be greater than this value.