Occupancy is closely related to the building energy consumption system and real-time monitoring occupancy is one of the hot issues in the study of building energy consumption behavior. Infrared sensing is a widely used method in existing research methods. Its measurement precision is related to installation height, personnel passing speed, personnel somatotype and other factors. In this study, the active infrared intrusion detector was selected to investigate the effects of installation height, passing speed and personnel somatotype on the measurement precision of occupancy according to the indoor occupants’ activities. The results show that the measurement precision first increased and then decreased with the decrease of the installation height when considering various personnel somatotype and passing speed. The slower the passing speed, the higher the measurement precision. When v≥1.4 m/s, the precision was less than 60%; when 1.0 m/s≤v<1.4 m/s, the precision was 70%~81%; when v<1.0 m/s, the precision was higher than 95%; When v<0.8 m/s, the precision was 100%. The height and body mass index(BMI) of the tested people were positively correlated with the measurement precision. Through the analysis of different operating conditions, an estimation method of comprehensive precision applied to actual scenes was proposed.