In this paper，the grain size of sediments and the aquifer vertical distribution were studied for borehole By1 in the North Jiangsu Plain. The grain size analyzing results indicate that there are five main aquifers bellow the depth of 20 meters and the ground water is held in silty sand layers. It demonstrates that there might be more ground waters at deeper depths. The aquifers in the study area was covered or protected by “hard clay layers”，which are widely distributed in the North Jiangsu Plain and were formed in the last glacial period，when the sea level was 150 meters below the present sea level. The protecting of the “hard clay layers” to prevent from sea water intrusion should be seriously considered during the exploitation of ground water in the North Jiangsu Plain at present.