Slag structure is important for the boron removal from silicon using slag refining. To study the correlation between slag structural and boron removal, the experiment of boron removal using CaO-SiOČ2-Na2O slags was carried out. Besides, slag structures were investigated by Raman and NMR spectroscopy. The results showed that when optical basicity increased from 0.6 to 0.66▒0.01, B2O3 could capture the non-bridging oxygen to incorporate into silicate network in the form of BO4 (1B, 3Si) and  B-3Si, which could reduce the activity of B2O3. Therefore, the boron removal rate increased with the increase of optical basicity. However, when optical basicity was increased to more than 0.66▒0.01, more Q2- could cut the Si-O-B key to separate the boron from silicon network, and the relative abundances of BO4 (1B, 3Si) decreased accordingly, which could lead to the increase of the activity of B2O3. Finally, the boron removal rate began to decrease.