To research the storage stability of composite coagulants PAC/PDM prepared using Polyaluminum Chloride (PAC) and Polydimethyldiallylammonium Chloride (PDM), Al-Ferron complexation timed spectrophotometry was used to test the changes of the species distribution of Aluminum in the composite coagulations before and after stored at room temperature or at 80℃ in water bath, and the jar test was also used to evaluate their variation of coagulation performance. The results showed that none obvious change of the species distribution of Aluminum in the composite coagulants, caused by stored in different time at room temperature or at 80℃,could been observed. The jar test results indicated that the obvious changes of removal efficiency in turbidity and CODMn of composite coagulants resulted from storage were still not measured. Therefore, composite coagulants PAC/PDM had behaved a good storage stability.