There is a class of problems in mathematics called the problem of the water pipe. The flow rate of the inlet pipe and the drain pipe being constants are not the case. By using the Bernoulli equation in fluid mechanics，we calculate drainage pipe flow along the variation of the water level of the pool and discuss in the inlet and outlet pipe being opened at the same time the conditions，the pool filled with water，and can not be filled with water conditions. The scientific errors in this kind of problem are corrected.