Abstract: Taking a Tonga MW8.0 earthquake as an example, we focus on the analysis of evolutional features of electromagnetic field spectrum in a band around cut-off frequency recorded by DEMETER satellite. The result indicates that the increase in electric spectrum amplitude were not closely related to earthquake preparation process based on comparison of 3-month data above the study region. However, we should pay more attention to the long period and large scale decreasing variation of the electric field in this frequency band. The image in local nighttime shows decreasing characteristic similar to that in local daytime. A statistical analysis on the data associated with 38 Mge;6.0 earthquakes in summer and autumn of 2006 demonstrates that for daytime data at the time 24 hours before the earthquakes electromagnetic emissions strengthened near the distance of about 2000 km, reaching 3 times RMS deviation (sigma;) relative to the background value, but weakened mostly in other time periods. While for nighttime data most anomalies above 3sigma; showed decrease whatever before or after the earthquakes. Finally, the earthquake-ionosphere coupling mechanism was discussed.