An oxygen-free condition is important and meaningful for the thermal plasma welding processes on aluminium and aluminium alloy because of the oxophilicity of materials. The addition of silane gas to the thermal plasma can reduce the oxygen partial pressure and thus contribute to a reduction in the thickness of the aluminum oxide layer in the melting zone by improving the quality of the welding processes. To understand the changes in the physical and chemical properties of argon or helium thermal plasma, especially after the addition of silane, optical emission spectroscopy (OES) was used to measure and analyze the spectra of the thermal plasma. The effect of silane on the changes of electron temperature and electron density of particles (e.g. Ar I and He I) in the thermal plasma was observed and discussed. Spectroscopic measurements were performed at different points in the direction of the arc column length of the non-transferred direct charge (DC) plasma and at different points in the direction of the arc column width. This allows the determination of the different distributions of electron temperature and electron density of the particles in the thermal plasma in the vertical and horizontal directions. In addition, the volume fractions of silane (argon or helium mixed with silane) were varied and the changes in electron temperature and electron density of Si I were observed, especially the electron density, to understand and discuss the reactivity of silane in different volume fractions in the thermal plasma.