In consideration of some key problems relating to the deformation of the Xiaowan Arch Dam during the first impoundment, the deformation was analyzed with qualitative and quantitative methods, including comprehensive hydrograph analysis, model analysis, numerical simulation, and mathematical statistics. The results show that the dam inclined upstream in the early stage of reservoir impoundment, mainly due to the sinking of the reservoir basin. The reservoir water level was the main factor in the dam deformation, and the time-effect component of the deformation tended to be convergent during the first impoundment. Comparisons of numerical results, modeled results, and designed warning indexes with measured data from the Xiawen Arch Dam, as well comparisons of its measured data with those of other similar projects, demonstrate that the Xiaowan Arch Dam operated normally at the normal water storage level. Although the deformation was relatively large in this case, its regularity was reasonable, demonstrating that the dam was in a safe condition during the first impoundment.