With the increase of prefabricated docks projects,the placement state of underwater piles is becoming the focus of attention.However,due to the isolation by seawater,there are relatively few efficient,low-cost and high-precision methods for monitoring the condition of piles.This paper proposes a method to obtain underwater pile point cloud by using multi-beam bathymetric system (MBS),which includes the GNSS RTK multi-beam sounding technology,the survey line layout taking into account the shapes and distributions of underwater piles and MBS measurement principle,and the clustering point cloud of the monitored pile by K-means++and the pile point cloud segmentation and filtering by considering the pile shape and MBS backscatter strength.The practical application results show that based on this method,the high-precision point cloud of the pile can be obtained,the state parameters of the pile are obtained,and the accuracy of the deviation in the x and y directions of the pile center is less than 5 cm,and the radius deviation is less than 4.5 cm,which verifies the correctness and effectiveness of this method.