In the evaluation of the operating range of an infrared imaging detection system, the average atmospheric transmittance is used, which is also a function of the operating range. Therefore, the operating range must be calculated using iteration algorithm. A variable step length method for the operating range evaluation of point target detection is introduced. MODTRAN software is used to calculate the atmospheric average transmittance and sky background radiance. The number of pixels, signal-to-noise ratio, and modulation contrast under the set distance are calculated using the evaluation model to identify whether the minimum performance index required for target detection is satisfied, and consequently determine the maximum operating range. When the difference between the set value and real operating range is large, a large step length is adopted; when the difference is small, a small step length is used. Compared with the const step length method, the proposed algorithm can accelerate the calculation significantly while maintaining accuracy.