In high speed continuous casting of peritectic steel slabs,mold fluxes with high basicity are required for less surface defect product.However,the basicity of remaining liquid slag film tends to decrease in casting process because of the crystallization of 3CaO·2SiO2·CaF2.Thus,a way is put forward to improve mold fluxes’ properties by raising the original basicity.In order to confirm the possibility of this method,the effect of rising original basicity on the properties of mold fluxes is discussed.Properties of high fluorine based mold fluxes with different basicities and contents of CaF2,Na2O,and MgO were measured,respectively.Then,properties of higher basicity mold fluxes were discussed and compared with traditional ones.The results show that increasing the basicity index can improve the melting and flow property of mold fluxes.With the increasing basicity,crystallization rate of mold fluxes increases obviously and crystallization temperature tends to decrease when the basicity exceeds 1.35.The method presented before is proved as a potential way to resolve the contradiction between horizontal heat transfer controlling and solidified shell lubricating for peritectic steel slab casting.But further study on improving the flow property of liquid slag is needed.This work can be used to guide mold fluxes design for high speed continuous casting of peritectic steel slabs. In high speed continuous casting of peritectic steel slabs, the basicity of remaining liquid slag film tends to decrease in casting process because of the crystallization of 3CaO · 2SiO2 · CaF2.Thus , a way is put forward to improve the mold fluxes’ properties by raising the original basicity. In order to confirm the possibility of this method, the effect of rising original basicity on the properties of mold fluxes is discussed. Properties of high fluorine based mold fluxes with different basicities and contents of CaF2, Na2O, and MgO were measured, respectively.Then, properties of higher basicity mold fluxes were discussed and compared with traditional ones. The results show that increasing the basicity index can improve the melting and flow property of molds fluxes.With the increasing basicity, crystallization rate of mold fluxes increases obviously and crystallization temperature tends to decrease when the basicity exc eeds 1.35 The method presented before is proved as a potential way to resolve the contradiction between horizontal heat transfer controlling and solidified shell lubricating for peritectic steel slab casting. Another research on improving the flow property of liquid slag is needed. This work can be used to guide mold fluxes design for high speed continuous casting of peritectic steel slabs.