Abstract: The silicon in high carbon metallurgical melt was tested with a developed Mg₂SiO₄(s)+MgO(s) silicon sensor. It was found that the phase construction of the auxiliary electrode and the oxygen component in the melt has an obvious effect on the measuring result. A physical model of this silicon sensor was proposed at the first time. The expression of local equilibrium silicon-oxygen potential, related to the oxygen potential of the melt and directly measured results, was derived. The physical meanings of coverage constant and structure constant were described. The structure constant was calculated to be 66.6 by means of fitting the experimental data. The measured oxygen potental, obtained with different silicon sensors whose coverage constant is less than 1, was corrected with the derived expression. The values of local equlibrium oxygen potential are the same. This testified that the physical model conforms to the practice.