Based on the data observed by French DEMETER satellite, thiS paper anatyzed anomalies which may be caused by Ms7.9 Tonga Islands earthquake （3 May 2006）. Perturbations of electric field and plasma parameters above epicenter from observations six days, two days and one day before the earthquake can be seen by visual interpretation of level-2 images. The amplitude and duration of perturbations increased when the earthquake was approaching. Perturbation was also recorded two days after the earthquake and its amplitude was still large. The variation feature of O^＋ density was more obvious than that of H^＋ and He^＋ density, and O^＋accounted for about 90% among total H^＋,He^＋ and O^＋through analyzing the data detected by IAP （Plasma Analyzer Instrument） onboard DEMETER satellite which has a revisiting period of sixteen days. Compared with the revisiting orbits of a half orbit one day before the earthquake from 2006 to 2008, it was found that variations of O^＋ density can be classified into three types： density in northern hemisphere is higher than that in southern hemisphere in summer; the situation is quite the contrary in winter; and O^＋ density is almost equal in both southern and northern hemisphere in equinoxes season. This paper presented different effects on O^＋ density under influence of earthquake and geomagnetic storm. The earthquake affects the distribution of O^＋ density at an orbit close to the epicenter, while geomagnetic storm affects O^＋ density at all orbits in a storm day. O^＋ density increased 20% more at the orbit closest to the epicenter than the other orbits one day before the earthquake, and it increased nearly an order of magnitude referred to the mean of O^＋ density in May from 2006 to 2008. In addition, we obtained electron density detected simultaneously by ISL onboard DEMETER at the above-mentioned revisiting orbits and discovered that electron density increased synchronously with O^＋ density. The increment of plasma density may be related to the electric field anomaly which resulted from the earthquake and led to modification of ionospheric structure.%利用DEMETER卫星观测数据，采用level-2图像目视解译的方法和重访轨道对比方法，对2006年5月3日发生在汤加群岛的7．9级地震进行分析．从震前6天、震前2天、震前1天和震后2天的level-2图像可看到，电场和等离子体参量在震中上空偏北的位置出现扰动信号，震前的扰动信号幅度随地震时间临近有所增强，扰动持续时间也相应增加；震后2天扰动信号依然存在，其幅度并未减弱．通过对比震前1天的轨道在2006年至2008年内的重访轨道，发现O^＋浓度存在三类较规律的变化特征，即在夏季时，北半球的值高于南半球；冬季时则相反；二分季节时，南北半球含量相当．本文给出了地震和磁暴对O^＋浓度的不同影响，地震影响最靠近震中位置轨道上的O^＋浓度，而磁暴影响全球轨道上O^＋浓度分布．同时获取了DEMETER卫星上朗缪尔探针在上述重访轨道同步探测的电子浓度数据，发现电子浓度与O^＋浓度存在同步增加．等离子体浓度增加的现象可能与地震产生异常电场进而影响电离层结构有关．
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