Abstract: In this article, the ultrahigh-order modes which exist in the symmetrical metal cladding optical waveguide are excited by using free-space coupling method. The refractive index and thickness of LiNbO3 slab are changed through adding different voltages between the two metal films which are in symmetrical metalcladding optical waveguide. The attenuated total reflection （ATR） spectrum with different parameters is obtained. The experimental results show that the ultrahigh-order mode is polarization independence and very sensitive to the thickness and refractive index of the waveguide layer. The experimental results agree better with theoretical predictions. Under the same conditions, the sensitivity of the symmetrical Al-cladding optical waveguide is higher than the symmetrical Au-cladding optical waveguide. The results have certain reference value for the development of the integrated optoelectronic devices.