Abstract: Valley deformation of the reservoir behind a high arch dam at initial impoundment stage affects the current working behaviors of the dam and its long-term safety. Monitoring the deformation of reservoir valley and hydraulic structures is crucial to engineering security. This paper presents a case study of the Xiluodu super-high arch dam project, which is subjected to a significant decrease in valley width at initial impoundment stage. For this reservoir, we obtained the field data of dam deformation in its abutment region collected by the monitoring system of its structures and foundation, along with the field data of valley deformation and its temporal and spatial variations. A preliminary analysis indicated that the dam body has been squeezed by its abutments. A multiple regression model of valley deformation was developed using the least-square method, and it was used to analyze the effects of reservoir stage, atmospheric temperature and time effect on valley deformation. Results show that time effect has been the major factor causing a large proportion of the valley deformation, rather than reservoir stage or temperature. At present the ongoing valley deformation of this reservoir is not yet finished and therefore a further study for mechanism analysis of the valley deformation is suggested in this paper.