Abstract: Pendulum wire is important in dam horizontal displacement monitoring.This paper discussed its attribute issues.It was pointed out that the dam deflection was in fact the dam horizontal displacement． Further, the necessity to correct the deformation monitoring precision in the criterion SDJ336－89 was stated here. The point view that section inclined deformation was not dam inclined deformation was advanced and monitoring by pendulum wire could provide conditions for single－parameter multi point numerical model analysis．