Abstract: The co-seismic response characteristics of the well water level in Fanxian County to the large teleseismic earthquakes are counted. Their response time, response amplitude, and response form are analyzed one by one. The mechanism of co-seismic response is discussed, and the rules are summarized. The results show that the observation well has a good ability to reflect earthquakes. The higher the earthquake magnitude, the observation well has the better ability to reflect earthquakes. From the perspective of the response amplitude, the co-seismic response between Ms6.5 and Ms7.5 earthquakes has obvious advantages, and the performance of continental earthquakes is better than that of non-continental earthquakes. Under certain conditions, the focal depth has a great influence on both the response time and the response amplitude. The co-seismic well water level response forms are not single, including impulse type, oscillation type and oscillation impulse type, and the response forms are related to the epicenter distance.