Operating distance is an important specification of infrared systems for detecting low temperature targets in space. By taking the noise of an infrared focal plane detector as the main factor, an operating distance estimation equation for point targets is derived. A calculation example of the operating distance of a typical infrared detection system is given. For the low temperature point target in space, the operating distance of the infrared detection system is verified by using both an energy equivalent method and a dimension equivalent method according to the signal-to-noise criterion in laborotary. The measured result is in agreement with the theoretical estimation. The test shows that the method can be used to evaluate the operating distance of an infrared detection system for detecting low temperature point targets in laboratory.