Many monitoring methods for dam safety have been proposed in recent years but it is difficult to monitor the arch dam automatically. A novel inspecting method for arch dam safety monitoring is described in this paper. This measurement system is based on the principle of transmitting the comparative displacement one by one. The error and accuracy of the system is analyzed. By this method, a large arch dam can be measured automatically. Long distance data transmission system using CAN bus and Internet is established. It is convenient to monitor the safety of dams from long distances.