Abstract: Blast furnace slag belongs to multiple sillicate system and can meltat high temperature. The material microstructure determines its macroscopic properties.This paper summarizes the research status of blast furnace slag microstructure,and analyzes their influence on slag fluidity and desulfurization ability.The chemical composition of blast furnace slag is mainly divided into modification and formation. Network structure in slag is mainly made up of silicon oxygen tetrahedron. CaO and MgO play their role as the network modification. With the increase of CaO and MgO content,the free oxygen ions concentration is increased in the slag,which destroys the silicate network structure and reduces the polymerization degree of complex anionic,thus the viscosity of slag reduces. But the excess alkalinity or MgO content also enhances the content of high melting point mineral in the slag,which makes the slag viscosity increase.Al₂O₃ and TiO₂ plays different roles in slag with different contents of the two material. Within a certain range of content,Al₂O₃ and TiO₂ acting as network modification,make the viscosity of slag reduce. Along with the increase of the content of Al₂O₃ and TiO₂,they convert to network formation and make slag viscosity increase,which worsens the liquidity of the slag. As MgO content or slag basicity increase,the complex ionic group is suffered,which improves the desulfurization reaction. But when the content of MgO is too high,the high melting point material increases,which leads to worsen the fluidity and lower desulfurization ability. Through studying the microstructure of blast furnace slag deeply,the relationship between the metallurgical performance and chemical composition of slag can be understood easily,which provides the theoretical basis for determining reasonable slag system.