X ray diffraction(XRD), scanning electron microscopy(SEM), thermogravimetric differential scanning calorimetry(TG DSC) and pore water expression(PWE) were used to investigate the hydration products and pore solution element concentration with different content of phosphogypsum in slight expansive road base cement. The results show that the amount of ettringite increases at first and then decreases in hardened cement paste with the increase of phosphogypsum content at 3d while it increases gradually with the increase of phosphogypsum at 28d. Sulfate generated by the dissolution of phosphogypsum has a good stimulation effect on the activity of fly ash. Al phase begins to dissolve largely after hydration for 7d. The concentration changes of Al, Ca and S in pore solution can reflect the hydration process of cement. The pH value of cement fluctuates slightly at 1255 with different phosphogypsum contents at different ages. After 60days of hydration, the hardened cement paste are denser and a large amount of ettringite still exists on the surface of the cement hydration products that contain a large amount of phosphogypsum, and there is more amount of needle like ettringite in the pores.