The setting time, strength and expansion properties of clinker steel slag fly ash phosphogypsum series road base composite binder were studied. The results show that the soluble impurities in phosphogypsum can react with Ca2+ and OH- to form insoluble substances covering the surface of binder particles, which significantly extends the setting time, the setting time can be prolonged about 65min with a 3% increase in the content of phosphogypsum.At the same time, large amount of phosphogypsum can provide sufficient SO2-4 ions for the slurry to ensure the large amount of stable formation of ettringite,and makes the hardened slurry slightly expand.However, excessive amount of phosphogypsum will also cause excessive expansion and leads to structural damage.The early hydration of composite binder and the optimization of pore structure can be promoted by adding appropriate amount of steel slag instead of fly ash,and the performance of road base composite binder can be significantly improved.