The detection of surface target objects is a prerequisite for unmanned ship to identify surface objects or avoid obstacles. Based on the absorption characteristics of laser signal by water body, a method of detecting surface target by 3 d laser scanning is designed in this paper. The method obtains the point cloud of the surface float by 3D laser scanning, then defines the effective point cloud set by conditional filtering, and divides the water surface target object by the Kd-tree based nearest neighbor clustering method, finally uses the axial bounding box(Axis Aligned Bounding Box, AABB)calculates the position of the target relative to the lidar. Three sets of surface object detection experiments are designed in this paper. The data shows that the method can detect the floating objects on the water surface and can complete the position measurement.