In the limestone/gypsum wet flue gas desulfurization pilot-scale test rig, key parameters such as SO2 absorption rate, mass transfer were experimentally determined．The results show that desulphurizing capacity of gypsum and fly ash is relatively weaker, which is only equivalent to fresh limestone with a content of 0.27% and 1.5% respectively. pH-t curve of slurry with different levels of fly ash could be divided into a sharply increasing stage and a steadily increasing stage. The leaching content of Mn2+ is about 9 times of Fe3+ , Mn2+ can form intermediate complex with HSO3- in the solution, which can induce catalytic reaction and accelerate SO2 absorption. Fly ash in gypsum slurry can obviously promote desulfurization. The pH value of slurry is high at the initial reaction stage, and effect of fly ash on SO2 absorption rate is less than 1.5%. when the pH value is decreased to 5.0, The leaching content of Mn2+ will grow with the decrease of pH value, better catalytic efficiency can be gained, effect of fly ash on SO2 absorption rate can increase 6.0% at most. The reaction is controlled by liquid phase resistance; the ratio of gas phase resistance to overall resistance is less than 38%. Mn2+ concentration of slurry increases with pH value decreasing and fly ash concentration increasing, which has significant effect on catalyzed oxidation of SO32-.