To improve the detection range of pyroelectric infrared sensor on the target (the pedestrian) , the passive infrared sensor detection system and IR signal were studied. The pyroelectric infrared sensor signal is affected by the perception parts on the target (pedestrians), temperature difference between the environment and objectives, lens size and sections related to the number. A detection system for testing infrared sensors distance were designed. Experiments showed that after taking into account several factors which affect the detection range, the detecting distance was improved from 10 meters to 40 meters while keeping the signal level of infrared detection unchanged. Test methods and results offer some guidance and reference for other similar detection systems.