Type Ⅱ anhydrite was prepared by calcination method and atmospheric acidification method using phosphogypsum as raw material.The differences of type Ⅱ anhydrite prepared by the two methods were analyzed and compared through the analysis of phase composition，micro-morphology，chemical composition and particle size distribution.The effects of different preparation methods on the setting time，hardening strength and hydration rate of type Ⅱ anhydrite were analyzed and compared under different dosage of alum.The results showed that the microstructure of type Ⅱ anhydrite AH-1 prepared by calcination was diamond shaped，and that of type Ⅱ anhydrite AH-2 prepared by atmospheric acidification was rectangular，in which AH-2 had high crystallinity and purity，small average particle size and large specific surface area.When the anhydrite was not mixed with alum，the hydration hardening was slow，the final coagulation time was more than 35 h，and the hydration rate was about 30% in 28 d.When the alum content was 1%（mass fraction），the final coagulation time of AH-1 was shortened from 40 h 20 min to 5 h 40 min，the final coagulation time of AH-2 was shortened from 35 h 20 min to 4 h 20 min，and the compressive strength of 28 d was 2.54 times and 3.01 times of that of the blank group，respectively.With the increase of alum content，the performance difference between AH-1 and AH-2 was gradually decreased.The final coagulation time difference between AH-1 and AH-2 was less than 30 min，and the 28 d compressive strength was 3.2 times that of the blank group.At the same time，the surface of the dihydrate gypsum produced by AH-2 hydration was smooth and the bonding was more dense.By comparison，it could be seen that the preparation process of type Ⅱ anhydrite by atmospheric pressure acidification was more advantageous.