By using data from CONOMOC ground-based GPS-TEC observations,we have applied the sliding interquartile range method to study the spatial-temporal distribution characteristics of ionospheric disturbances of seven M_S≥6.0 earthquakes occurred in continental China during 2008～2019. Our results show that the GPS-TEC values at observation stations show negative anomalies for five earthquake cases of seven events. TEC anomalies mainly appeared two to six days before the earthquakes. The ionospheric TEC positive abnormal disturbance during the earthquake,mainly concentrated two to seven days after the earthquake. The closer the station is to the epicenter,the larger the TEC anomalies becomes;during a few earthquakes TEC values increased significantly. Most of the earthquake cases show that and the closer the station is to the epicenter,the more obvious the TEC disturbance can be detected over the epicenter,with the maximum disturbance range of as much as 2000km away from the epicenter. With the increase of the magnitude of the earthquake,the occurrence rate of the ionospheric TEC anomaly before the earthquake is increased,and the coverage of the anomaly is also expanded. The results show that it is possible to identify ionospheric anomalies before earthquakes from continuous GPS observation data analysis. Therefore,we propose that monitoring ionospheric TEC changes within a week before the earthquake may be a clue to the relationship between ionospheric disturbances and seismic activities.