The phosphorus enrichment behavior in dephosphorization slag was investigated by slag modification with Al 2 O 3 in the present study. The mineral phase of the slag samples were measured by mineral phase microscope, SEM + EDS and XRD. The mechanism of Al 2 O 3 modification was discussed according to FactSage calculation. The results show that phosphorus was mainly existed in nC 2 S–C 3 P solid solution and Al 2 O 3 modification was beneficial to the enrichment of phosphorus. The content of phosphorus in phos-phorus-rich phase was increased to 5.10% and 9.15% with 8% and 11% Al 2 O 3 addition. With the slag temperature decreases, Ca 3 (PO 4 ) 2 and Ca 2 SiO 4 firstly precipitated and formed the nC 2 P–C 3 P solid solution. The addition of Al 2 O 3 has a positive influence on the reaction of Al 2 O 3 and nC 2 S–C 3 P solid solution. Al 2 O 3 could react with the initially precipitated low phosphorus nC 2 S–C 3 P solid solution to produce higher phosphorus n ′ C 2 S–C 3 P solid solution (n ′ <n) and Ca 2 Al 2 SiO 7 . When Al 2 O 3 content was increased from 2% to 15%, the value of lnK decreased from 2.34 to 1.47.