It is considered that the shost-period variations of electro telluric field at Daishan middle school station belong to the geodetic potential through the analyses of parallel direction correlation, the pure anomaly vector ratio, P_C electrotelluric pulsation, geoelectric bay, etc. The station is highly sensitive to geodetic potential change. In addition, the characters of geodetic potential anomaly before the strong earthquakes occurring in Lancang, Batang, etc. are analyzed. These anomaly variations affected by the ultra-low frequency electromagnetic radiation occurred within 8.8 -32.5 hours before earthquakes and lasted the time from 20 min to 75 min.