In infrared imaging system, all false and unnecessary signal are called noise. The existence of the noise
interferes detection for useful signal, and affects system operating range. Starting from
the statistic characteristics of the signal and noise, the relationship between
false-alarm probability and threshold SNR is deduced, as well as detection probability and
SNR. The formula of operating range for spot target is given. As for extended target, the
concept of temperature difference is redefined, and the formula of calculating system operating range based on MRTD is given, considering of the dimension of the target, discrimination level and detection probability. A software is developed which can fast calculate the operating range under different detection probabilities. At last, an example is given.