Too thick siltation deposit of foundation trench under water may cause more settlement and even slide of ground. No detection methods for thickness of siltation deposit are proposed by present codes. Many kinds of detection methods are used for engineering and some methods are improper. Basing on distinguishing siltation deposit from fluid mud, the present possible detection methods are sorted, introduced simply and evaluated. Finally, the importance of distinguishing siltation deposit from fluid mud and choosing proper detection method is pointed out through a project.