Before and after the Datong-Yanggao earthquake in October 1989, a high precision ther-mometer (resolution 0.0001℃) installed at the Sanmafang Ooservatory, which is about 75 km from the epicenter, recorded a significant geothermal anomaly. The amplitude of the anomaly is 0.15℃ and the duration time is over 50 days. This paper discusses the characteristics of the abnormal temperature variation. As a whole, the variation has a shape of increasing temperature with a type of the power function, and is a temperature effect due to the fault creeping caused by the increase of regional stress field. Several days before the earthquake, the geothermal precursory anomaly showed a process that is, "declining-decline rate slowing down-sharply rising-short-term stableness-an earthquake occurring". The mechanism of the geothermal anomaly is also studied, preliminarily.