Abstract: A silicon sensor auxiliary electrode was successfully synthesized by evenly coating CaF₂+SiO₂ powder on the surface of ZrO₂(MgO) solid electrolyte and roasting at 1400 ℃ for 30 min in high purity Ar atmosphere. The effects of preparing conditions on the composition,crystal phase and morphology of the formed auxiliary electrode membrane after calcination were systematically investigated by X-ray diffraction,scanning electron microscopy and energy dispersive spectrometry. It is shown that SiO₂ solid particles,CaO·MgO·2SiO₂ solid solution and ZrSiO₄ are the main components of the formed auxiliary electrode layer and no CaF₂ exists in the layer. In addition,the influences of phase transformation on the film adhesive property and silicon sensor performance were also discussed. The as-prepared silicon sensor which is used to detect silicon content in molten iron at 1450 ℃ shows a better performance.When the silicon content ranges from 0.5% to 1.5% in liquid iron,the testing result based on this silicon sensor with a response time of 10 s and a steady time more than 20 s agrees well with the value based on the chemical analysis method.