Dam-reservoir interaction was analyzed in this paper using fluid-  solid coupling method. The reservoir water was modeled respectively by two types of finite element, namely pressure-based element and displacement-based element. The fluid-solid coupling method was compared with traditional Westergaard additional mass method and the comparison showed that the former was closer to the theoretical solution than the latter. The displacement  based element demanded more regular element shape than pressure  based element. Moreover, sensitivity analysis of selected areas of the reservoir showed that satisfactory results can be obtained if the length of reservoir area to be modeled was designed only 3-5 times the height of the dam. Finally, the compressibility of water was also explored taking the gravity dam structured model with different dam crest widths as an example. The present results will be helpful for the following research.