A lot of experiments have proved that there is an obvious abrupt geothermal anomaly in volcanic region before volcanic eruption. The characters of the abrupt geothermal anomaly concern many factors such as geological structure, cause of volcanic formation, volcanic scope and type. On the other hand, this kind of geothermal anomaly is an important sign for using thermal infrared remote sensing to monitor and forecast the volcanic activities. This paper analyses rising temperature's mechanism on the earth's surface in the anomalous region from radiation, convection and so on, and raises an atmospheric transmission equation of thermal infrared radiation on the basis of surface feature effective radiation when thermal infrared radiation transmits and the cover of earth's surface feature, air, cloud, rain, wind and cold airflow affect it. All of noise analysis in the article are the theory foundation for using thermal infrared technology to monitor and forecast volcanic activities.