尾水管（英语：Draft tube）是一种安装于像是反击式、卡布兰式或是冲击式水轮机水流出口的一种扩压管路，尾水管是宽度渐宽的管路，因此速度会变慢，但压力会随之提高。

在冲击式的水轮机中，它的有效可用水头高度可以相当高，如果水轮机安装位置与尾水放水路高度之间仅有几米的话，在运转效率上的优化并不明显。然而，在反击式水轮机中，如果净水头高度极低，并且水轮机安装位置高于尾水高度，其有效水压水头高度会有相当的损失。如果水流在水轮机出口处的压力比在尾水口还低，水流将会逆流回到水轮机中而损坏。透过将扩压管安装在水轮机转轮出口处，可以降低出口水流速度，因此提高压力水头高度，可以有效提升整体运转效率及水轮机输出性能。

In power turbines like reaction turbines, Kaplan turbines, or impulse turbines, a diffuser tube is installed at the exit of the runner, known as draft tube. In an impulse turbine the available head is high and there is no significant effect on the efficiency if the turbine is placed a couple of meters above the tail race. But in the case of reaction turbines, if the net head is low and if the turbine is installed above the tail race, there can be appreciable loss in available pressure head. If the pressure at the exit of the turbine is lower than the pressure of fluid in the tail race, a back flow of liquid into the turbine can result in significant damage. By placing a diffusing pipe at the exit of the runner, the pressure head is increased by decreasing the exit velocity, and both the overall efficiency and the output of the turbine can be improved.