Abstract: Since 80′s, a very uniform fine sand layer has deposited, with the thickness of 10 m or more, in the Three Gorges Area of Yangtze River. The dynamic stability of that fine sand has come to the attention of engineering constructers.In this paper, the dynamic characteristics of the fine sand under the earthquake and/or the excavation blasting effect are studied, and those impacts on the high earthrock cofferdam is considered. According to both the lab and insitu experiments and measurements and to numerical analysis as well, this sand layer might readily be liquified where the overlying load is relatively low or where the ground is loadfree.It will weaken the stability of cofferdam.As for the problem of blast loading, it seems to have a lower bearing on the project than a grade 7 earthquake does.