Abstract: This paper analyses the cause of doubtful point on deformation which has great difference between the horizontal displacement values on the crest of the No.9 dam block measured by plumbline of PL and collimating line of AL6 from the view of observation error and temperature-loading. It indicates that the main cause of these difference, through measurement of height difference on those two points and horizontal distance change, are contributed to the upper pier concrete deformation developed as the temperature changed and accumulated to the pier end, then result in the measurement difference between those two points on the top of pier.