Occupancy is closely related to the building energy consumption system. Monitoring occupancy is one of the hot issues in the direction 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. The active infrared intrusion detector was selected to study the influence of installation height, passing speed and personnel somatotype on the measurement precision of occupancy according to the indoor occupants" activities. The research results show that the measurement precision first increases and then decreases 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 ≥ 1.4m/s, the precision is less than 60 %; when 1.0 m/s ≤ v < 1.4 m/s, the precision is 70 % ~ 81 %; when v < 1m/s, the precision is higher than 95 %; When v < 0.8m/s, the precision is 100 %. The height and Body Mass Index of the tested people are positively correlated with the measurement precision. Through the analysis of different operating conditions, an estimation method of comprehensive precision applied to actual scenes is proposed.