The invention discloses a water return system of a phosphogypsum slag dump and belongs to the technical field of phosphoric acid production. The water return system of the phosphogypsum slag dump comprises a slag dump body, a seepage water collecting groove, a collecting water pool and a conveying pipeline network. The conveying pipeline network is connected with at least one phosphoric acid production system and comprises at least one group of conveying pipelines. Each group of conveying pipelines are connected with the collecting water pool and the corresponding phosphoric acid production system and comprise a non-variable-frequency pump pipeline and a variable-frequency pump pipeline, wherein a non-variable-frequency pump is arranged on the non-variable-frequency pump pipeline, and a variable-frequency pump is arranged on the variable-frequency pump pipeline. Water return pipelines are connected between the two ends of each non-variable-frequency pump pipeline and between the two ends of each variable-frequency pump pipeline respectively. A pressure release valve is arranged on each water return pipeline. According to the water return system, each group of conveying pipelines are formed by one variable-frequency pump and one non-variable-frequency pump, the flow is controllable, and the structure that one pump is in use and the other pump is used for standby application is formed; in addition, by arranging the water return pipelines provided with the pressure release valves, it is guaranteed that the pressure of each pipeline does not exceed the standard; furthermore, water return balance is guaranteed by setting the sizes of the valves.