Abstract: With the development of computer network technology, artificial intelligence, images recognition and decision support system, precision farming technology system is used more and more widely. In this paper, image information processing technology such as mathematical morphology and watershed segmentation algorithm is used to monitor pest population density. According to the distance between CCD and flying insects and size of insect individuals, watershed segmentation algorithm based on priori information is proposed, which can effectively restrain the impact of background and insects’ wings, while at the same time raises the efficiency and precision of pest data acquisition greatly.