Impurities such as residual acid and soluble phosphorus content in phosphogypsum have a great impact on the ratio and strength development of phosphogypsum cementitious material. Two different qualities of phosphogypsum were used to composite with slag and cement to prepare cementitious materials with phosphogypsum content of 60%, 70%, and 80% (mass fraction), and wet grinding modification was carried out on phosphogypsum. By testing the pH value of the pore solution of the cementitious material, the degree of slag hydration, and compressive strength, and combining XRD, SEM-EDS and other testing methods, the influence of soluble phosphorus impurities on the hydration process of high dosage phosphogypsum cementitious materials was explored. The results show that the optimal dosage of Portland cement in high dosage phosphogypsum cementitious material is related to the soluble phosphorus impurities contained in phosphogypsum. The higher the content of impurities, the higher the content of cement. After wet grinding pretreatment, 7 d strength of the cementitious material can be improved, but cannot increase the 28 d strength. The strength of the composite cementitious material depends on the degree of slag hydration. During the hydration process, the soluble phosphorus in phosphogypsum and the Ca(OH)2 hydrated by cement form calcium phosphate precipitates on the surface of the slag, which hinders the hydration of the slag and is the main reason affecting the development of its strength.