In order to find the method of auto detecting body temperature in pig’s cultivation, the homemade thermal infrared imager was used to acquire the infrared thermograms of pigs. The imager was loaded on the arm of the auto-inspection trolley with the camera lens toward pigs, so it can clearly collect the images of pig ear roots. When the thermograms were collected, they were firstly transformed by HSV colorspace into the S-layer images. Furthermore, the morphological closing operation was used for binary image denoising. Finally, adapted Otsu was applied to ear roots detection of piglets, finishing pigs and pregnant pigs separately. The objective function was selected, because it was simple to find a grayscale to maximize η with the minimum computation at the same time. The results showed that the method can be perfectly used to analyze images with complete ear roots for piglets, finishing pigs and pregnant pigs. When the method was applied to those images with incomplete ear roots, the detection rates were 23% for piglets, 25% for finishing pigs and 33% for pregnant pigs.The method cannot be used to analyze images without any ear roots of pigs. The method was applied to the auto-inspection system which can help to find the high temperature pigs in the cultivation. When the abnormal pigs were found, the auto inspection trolley would alert, and then stopped inspecting and began to spray.