In order to improve the dephosphorization efficiency of low-basicity converter slag and decrease the consumption of solid CaO, the effect of high Na2O addition (0–15 mass%) on the distribution of P2O5 between solid solution and matrix phase was investigated. The thermodynamic properties of slag samples were calculated by FactSage 7.0 software. Then, the dissolution behavior of phosphorus element from slag was studied. The results show that the mass fraction of 2CaO·SiO2–2CaO·Na2O·P2O5 solid solution increases when the Na2O content is increased in slag. However, the amount of formed solid solution changes little when the Na2O addition is more than 10 mass%. Moreover, the content of Na2O in solid solution would reach saturation when adding 15 mass% Na2O into the slag. With the increase in Na2O content in slag, the distribution ratio of P2O5 between solid solution and matrix phase increases. The values are evidently higher than the results reported in the literature. Meanwhile, the activity of P2O5 in matrix phase and the activity coefficient of P2O5 in solid solution are decreased with increasing the Na2O content in slag. Furthermore, the dissolution ratio of phosphorus in citric acid solution could be improved by adding Na2O into slag, but the increment of dissolution ratio would decrease when the Na2O content exceeds 10 mass% in slag.