Infrared sensor by detecting the infrared radiation, within its field of view, the number of people can be determined through effective detection of indoor temperature. This paper designed a system that based on MLX90621 infrared sensor to detect the number of indoor people, in term of using Ostu to determine the threshold,caused a serious misjudgement on image segment,a new fixed threshold segmentation based on histogram statistics and multiframe sampling is proposed.According to the results of the histogram statistics,we get a fixed global threshold for the binarization processing,and then use the method of multiframe sampling and comparison to detect the number of indoor effectively.