Abstract: In recent fifteen years, bridge collapse causes economic losses and casualties frequently, of which nearly 60% are caused by underwater structural diseases. The slope of piers and piles of bridges is the main reason of bridge collapse. If the slope of piers is detected in advance, the accident rate will greatly decline for timely maintenance. However, manual detection is still the main way to detect the diseases and measure the slope of bridge underwater structures in China. This paper proposes a positioning slope measurement method based on sonar, which is verified by practical engineering project. The implementation steps are using the two-dimensional sonar carried by ROV to obtain planar imaging cloud maps at different depths of water, fitting the edges and geometric centers of the pier columns, and calculating the slope of pier pile.