Abstract: Preparation of calcined phosphogypsum (CPG) is an important utilization way for phosphogypsum (PG) on a large scale. In this paper, the effects of grinding fineness on the phase transformation rate, mechanical strength and structure of hardened body of the CPG were studied. The results show that increasing calcination temperature is beneficial to accelerating the dehydration of PG, the appropriate calcination temperature is 120℃. With the increase of grinding fineness, the conversion rate of PG into CPG first increases and then decreases; the flexural strength and compressive strength of the prepared CPG first increase and then decrease; the hardened body of the CPG prepared from non-grinding PG has loose structure, many holes and serious pulverization, and the flexural strength and compressive strength are only 0.24 MPa and 0.57 MPa, respectively. When the average particle size of PG is reduced to 49.95 μm, the dehydration rate is accelerated, the crystal size of dihydrate gypsum is uniform, the hardened structure is dense, and the pores are less, the flexural strength and compressive strength are increased to 1.02 MPa and 2.62 MPa, respectively. Therefore, grinding modification is not only beneficial to increasing the dehydration rate of PG, but also to effectively improving the structure of hardened gypsum and increasing the mechanical strength of CPG.