This paper presents a new method to monitor the deformation of dams.Using this measuring scheme the horizontal displacement of all the portions of dams can be measured simultaneously in the instant moment.To get an extremely accurate direction a beam of laser is used as a reference line.Our measuring system has many advantages over the others:1)It can measure the displacement of the entire dam.2)It can dramatically diminish the measuring time.3)It can provide a real time displacement of dams.4)Its accuracy surpasses 0.1 mm.