In recent years，there have been more and more environmental protection dredging projects in rivers and reservoirs and port and waterway construction and maintenance dredging projects. Pipeline transportation is the main method of dredging sediment transportation，and the transportation distance is also getting longer and longer. The redistribution of the flow velocity and the critical flow velocity along the pipeline needs to be further studied. The model is calibrated based on the measured data of the clean water conditions and the stratified sampling concentration of the sand transport conditions of the Gulei sand filling project. On this basis，three-dimensional hydrodynamic and sand motion analysis are performed on the long straight pipeline transportation of sand particles. The redistribution characteristics of the section velocity along the straight pipeline and the law of the critical velocity of sediment deposition are analyzed. The formula of the critical velocity of sediment deposition at different sections in the long pipe is put forward，which provides a reference for the design of dredging projects.