Abstract: The effect of valley width deformation during the initial impoundment stage on the subsequent working behavior and the long-term safety of an arch dam has become an increasingly important chanllenge faced by dam engineering and academia. This study aims to analyze the sensitivity of valley width deformation to the absorption curve of fractured rock mass to address the challenge of valley width reduction during impoundment of the Jinping Ⅰ arch dam in China. The methods of unsaturated seepage analysis theory and nonlinear finite element numerical analysis were applied to study the law of valley width deformation during unsaturated seepage process. The influence of valley width deformation on the displacement and stress of the dam was analyzed. The results showed that the slope on both sides of the valley deformed towards the center under the action of the unsaturated seepage field. The deformation of valley width upstream exceeded that downstream. The valley width deformation increased with an increasing elevation of the water level, and maximum valley width reduction occurred in the saturated seepage field. The distribution of displacement and stress of the dam changed little during the unsaturated seepage process. However, the maximum longitudinal displacement and the minor principal stress of the dam decreased gradually with increaseing elevation of the water level. There was a gradul increase in the major principal stress. The valley width reduction had a squeezing effect on the dam body, resulting in a decrease in the maximum longitudinal displacement, the transfer of the major principal tensile stress from the dam heel to the upstream side of the dam abutment, and the extension of the high-pressure stress zone on the downstream surface to the middle of the crown cantilever. The influence of the saturated seepage field on the displacement and stress of the dam was more obvious than that of the unsaturated seepage field. However, the valley width deformation resulting from the seepage field had a limited effect on the displacement and stress of the dam, which would not affect the overall stability of the dam.