Groundwater age is an important and widely used hydrogeological parameter. The environmental isotope method is the most reliable method of groundwater dating. Different environmental isotopic tracers have different dating ranges, and the determination of groundwater age from to 50~1 000 a is the “short board” of dating groundwater. The period of 50~1 000 a is a period of frequent human activity. Therefore, dating groundwater that is 50~1 000 years old is scientifically significant. Silicon-32 (³²Si) can be used to determine the age of 50~1 000 a groundwater, but application of ³²Si dating is hindered by the complex and time-consuming pretreatment. The decay of ³²Si to radiogenic ³²P, ³²Si, and ³²P will reach a radioactive balance within three months and achieve consistent radioactive activity. ³²Si and ³²P in 50~1 000 a groundwater have been in radioactive equilibrium, so direct enrichment of ³²P in groundwater can be used for age determination of 50~1 000 a groundwater. Magnesium hydroxide co-precipitation is used for the enrichment of ³²P in groundwater. This method is convenient and rapid. Therefore, ³²P is expected to solve the dating deficiency of 50~1 000 a groundwater. The results of ³²P dating of groundwater in the Jianghan Plain were roughly consistent with those of ³²Si. Therefore, radiogenic ³²P groundwater dating is feasible, convenient, and accurate.