The refining of MG-silicon (MG-Si) is closely related to the cost and purity of solar-grade silicon (SoG-Si) as well as semiconductor-grade silicon (SeG-Si). Plasma arc refining of MG-silicon is one of the alternative and effective route to remove the impurities in silicon. In this study, a 60KW transfer-arc plasma melting furnace operated in 10(5) Pa was used to purify the MG-Si by different kinds of working gas, which was composed of 100%Ar, 95%Ar+5%O-2, 95%Ar+5%H-2, and 70%Ar+30%H-2 respectively. During the processing, an optical spectrometer was used to monitor the changes of compositions. The experimental results show that the removal rate of impurities of aluminum, calcium, sodium, barium... etc. in silicon with plasma working gas containing oxygen, and hydrogen are higher than pure Ar plasma. Especially with 30% H-2 plasma, the removal rate of the Na and Ba could reach 100% and the removal rate of Ca and Al could also achieve to 99.5% and 89.5% respectively. For the impurities of boron in the MG-Si, the elimination rate of hydrogen-mixed plasma could be as high as 76%. The in-situ monitoring of plasma refining is accomplished with the monochromators in the range of visible light's wavelength. From the results of chemical analysis and optical spectrograph, it revealed that elimination rate of Fe and Al was higher in hydrogen-contained plasma arc than in pure Ar plasma, As to the refining of carbon, the hydrogen and oxygen mixed plasma arc are also efficient to reduce the carbon content in silicon, which could be decreased from 310 ppm to 60-70 ppm.