Four kinds of phosphogypsum with different particle sizes were selected and mixed with mechanical sand, cement and admixtures to prepare wet-mixed mortar. The effects of phosphogypsum particle size on the workability and mechanical properties of mortar were investigated, and the mechanism of phosphogypsum particle size on the hydration products and microstructure of wet-mixed mortar was investigated by XRD, TG-DSC, MIP and SEM test. The results show that with the increase of phosphogypsum particle size, the workability and mechanical properties of wet-mixed mortar show a trend of increase first and then decrease. When 30% (mass fraction) phosphogypsum with particle size of 53~106 μm is added, the wet-mixed mortar consistency loss is 19%, water retention rate is 90%, 28 d compressive strength is 10.7 MPa, 14 d tensile bond strength is 0.25 MPa, which can meet the requirements of compressive strength greater than 10 MPa. With the increase of phosphogypsum particle size, the eutectic phosphorus content in phosphogypsum decreases, the hydration process of cement is inhibited to a lesser extent, the calcium silicate hydrate (C-S-H) generated in mortar increases, and a large amount of C-S-H appears in the region away from CaSO4·2H2O particles.However, the pore volume of mortar hardening body shows a trend of decrease first and then increase, when 30% (mass fraction) phosphogypsum with particle size of 53~106 μm is added, the pore volume of mortar is the smallest, only 0.130 9 mL/g. Phosphogypsum particle size range is suitable to be controlled in 53~106 μm, at this time, the wet-mixed mortar has good workability and mechanical properties.