Sn-Si alloy was refined for removal of boron by composite solvent refining with CaO-SiO2-Al2O3 slag at 1550℃, and the effects of optical basicity of slag and alloy composition on the removal of B and refining mechanism were examined. The results showed that with increasing of the optical basicity, the activity coefficient of BO1.5 decreased obviously, the partition ratio of B increased, leading to the enhancement of refining. When the content of Sn in the alloy increased from 0 to 70%, the partition ratio of B increased from 3.16 to 13.8, the lowest content of B was 0.89′10-6, and the highest removal rate of 93.3% achieved. When the content of Sn was over 30%, the viscosity of Sn-Si alloy substantially decreased, resulting in rising of the partition ratio of B. When the content of Sn was 70% and the viscosity of alloy 0.61 mPa×s, the highest values of partition ratio of B in both CaO-SiO2-20% Al2O3 and CaO-SiO2-40% Al2O3 slag systems were 10.1 and 12.3, respectively.