The phosphorus enrichment behavior in dephosphorization slag was investigated by slag modification with Al₂O₃ in the present study. The mineral phase of the slag samples were measured by mineral phase microscope, SEM+EDS and XRD. The mechanism of Al₂O₃ modification was discussed according to FactSage calculation. The results show that phosphorus was mainly existed in nC₂S–C₃P solid solution and Al₂O₃ modification was beneficial to the enrichment of phosphorus. The content of phosphorus in phosphorus-rich phase was increased to 5.10% and 9.15% with 8% and 11% Al₂O₃ addition. With the slag temperature decreases, Ca₃(PO₄)₂ and Ca₂SiO₄ firstly precipitated and formed the nC₂P–C₃P solid solution. The addition of Al₂O₃ has a positive influence on the reaction of Al₂O₃ and nC₂S–C₃P solid solution. Al₂O₃ could react with the initially precipitated low phosphorus nC₂S–C₃P solid solution to produce higher phosphorus n′C₂S–C₃P solid solution (n′<n) and Ca₂Al₂SiO₇. When Al₂O₃ content was increased from 2% to 15%, the value of lnK decreased from 2.34 to 1.47.