In this paper,the minute data observed from January to December 2011 by Gaoyou,Xinyi,Lingyang and Changli electric field stations in eastern inshore areas of Chinese mainland were processed via the wavelet transform,and were compared to the Datong,Shizuishan and Gufeng stations in the inland areas. We analyzed the change of amplitudes of semidiurnal variation. The results showed that:(1)The phenomenon of the increase and decrease of diurnal variation amplitude of semidiurnal wave occurs twice for each month in part of months in 2011 at Gaoyou,Xinyi,Lingyang and Changli electric field stations. This phenomenon in different months at the station occurs almost in the same time(lunar calendar date)and in the same month at different stations occurs in the different times,but the stations in the inland areas didn't observe the phenomenon. (2)The relationship between the condition of stations in measured area and this phenomenon is studied. A preliminary conclusion is drawn that this phenomenon has connection with the river drainage systems in measured area and near. The occurrence time of this phenomenon may be associated with the magnetic latitude of the stations. (3)The possible cause of this phenomenon is discussed.