The recycle and reuse of desulfurization gypsum as one of the largest solid emissions from power plants was important to environmental protection.With dry process desulfurization gypsum of the power plant as raw material，dihydrate gyps um（CaSO4·2H2O） was prepared by oxidizing and recrystallizing calcium sulfite for use as cement retarder.The influences of factors，such as pH of solution，ratio of liquid to solid（mass ratio），amount of catalyst，air velocity and reaction time，on oxidation conversion of calcium sulfite were investigated.The optimum conditions of prepared CaSO4·2H2O from oxidized calcium sulfite were confirmed as follows: the pH of solution was at 4~5，the reaction time was 1.5 h，the amount of catalyst（the mass ratio of calcium sulfite to catalyst） was 7.5，the air velocity was 0.15 m3/h and the ratio of liquid to solid was 10∶1.Under the conditions，the oxidation conversion of calcium sulfite was about 100%.The CaSO4·2H2O product could be further used in industrial production with nice crystal forms and good filterability.