Detection probability,false alarm probability and detection distance are the most important performances of the infrared detection system. Traditional performance analysis methods only consider single infrared detection system. As the co-detection applications of several infrared detecting systems increase,performance analysis method of co-detection is urgently necessary. Starting from the classic equations of detection probability,false alarm probability and detection distance for single infrared detection system,the root cause of performance improvement for co-detection is analyzed based on “two-time accumulation detection” theory. Taking co-detection of two infrared detecting systems as an example,the improved effect of co-detection is demonstrated. This provides an important reference for the demonstration and co-detection pattern analysis of infrared detection systems.