Abstract: In this paper the quantitative relationship between water head in a model of horizontal multilayer aquifer and stress in aquifer has been deduced. Based on the observational data of co-seismic step variations of water level at 52 wells at the time of the Akita earthquake,Japan,the adjustment variation of co-seismic stress field in China has been calculated by means of the effect of bulk strain tide on water level in wells. The probable origin that areas of stress increment alternate with areas of stress decrement during the stress adjustment in China has also been discussed.