In order to improve the detection sensitivity, the method of dual-wavelength differential detection is used. To realize the measured refractive index range being adjustable, a novel surface plasmon resonance sensing structure is described, which consists of a planar waveguide and a symmetric structure with metal-sample-metal layer. The mode characteristic of the symmetric structure is analyzed. When the surface plasmon wave is excited, the relationship between the tested refractive index and the dielectric thickness, and the dual-wavelength differential method are researched. Planar waveguides are prepared by the ion-exchange and the graded-index distribution can be fitted by the Fermi function. In experiments Glycerin solutions with the refractive index range of 1.33-1.428 are used for detection by intensity method. Methods of single wavelength and dual-wavelength based on the symmetric structure are used respectively, then the plasmon surface wave can be excited. The refractive index matching condition of this structure can be adjusted through the thickness of the sample, so the tested range is adjustable. If the measured range is appropriate, a linear relationship between normalized intensity and refractive index can be obtained. Theoretical analysis and experimental results show that the sensitivity of the differential detection method is nearly doubled compared with the single-wavelength method.