Abstract: Method, instruments used and their improvement in deformation measurement of concrete dam are introduced and a commentary on all kinds of the method and instruments is presented. In the paper, dam monitoring network is expounded in detail and its important function in dam safety monitoring and some steps for improving accuracy of the measurement are analyzed. It is considered that plumb line deflection will influence the measurement results. An analysis about the influence is presented so that researchers in the measurement field can pay attention to the influence.