The influence of Cs 2 O-containing refining slag on the purity of steel for ultra-fine saw wire has been studied in this article. The result indicated that a small amount of Cs 2 O can significantly improve the capability of low-basicity refining slag to purify molten steel. In detail, the number and size of inclusions decreased dramatically when Cs 2 O was added to the refining slag. Additionally, the composition of most inclusions is controlled within the low melting point region. However, it will reduce the capability of low-basicity refining slag to purify molten steel if the content of Cs 2 O exceeds 10 wt%. Cs 2 O changes the ability of low-basicity refining slag to remove inclusions by changing the slag's viscosity and microstructure. The viscosity of refining slag increased gradually with the mass fraction of Cs 2 O in the slag increasing, and the disparity of viscosity values between different experimental groups increased sharply with temperature decreasing. Furthermore, the viscosity curve of Cs 2 O-containing slag developed a turning point, that is, the speed of viscosity increasing raised sharply at the turning point, while this phenomenon cannot be found in a normal experiment group without Cs 2 O. The detection results of the Raman spectrum show that the three-dimensional network of slag increased greatly with the content of Cs 2 O in the slag increasing gradually.