Abstract: In this paper, we use the geomagnetic analysis " The difference in daily amplitude” method to analyze the characteristics of " The difference in daily amplitude” based on the wavelet analysis of geomagnetic observatories in Jingyang, Lianyungang and Xinyi before the Wenchuan earthquake. The results show that there is anomalous variation of " The difference in daily amplitude”ΔT_g (24) before the Wenchuan M8.0 earthquake and the time of the anomalous variation is 150 days and 25 days before the earthquake, although there is no such variation between Xinyi and Lianyungang. This indicates the Wenchuan 8.0 sesimo-magnetic effect is likely to have an effect within 700 km, but its suffering area is not able to reach 1 500 km. The results show that wavelet analysis is an effective method to extract seismic magnetic effect.