A symmetric structure of surface plasmon resonance excited by planar waveguide is studied.The structure is the metal layer covering both sides of the media with a certain thickness.The theoretical analysis shows that the TM0 mode of that structure is surface wave and the relationship between the equivalent refractive index and the interval of metal films is studied under a certain wavelength.Ionexchange technology is used to prepare the multi-mode planar waveguide.The planar waveguide is used to excite the plasmon surface wave of the symmetric structure and its equivalent refractive index is tested.Plasma surface wave is excited when the concentration of glycerin solution is 0-70% and the thickness is 300 nm and 500 nm respectively.Results show that the refractive index matching of this structure can be adjusted by changing the thickness of media.The problem that the equivalent refractive index of ion-exchange planar waveguide is large and it is difficult to excite the plasmon surface wave between the metal and the media to achieve sensing is solved.