An optimal structure of on-line optical fiber surface plasma resonance (SPR) sensors is proposed based on principle of frequency modulation. On the basis of theoretical analysis, numerical simulation and measured results, the optical fiber SPR sensing probes can be regarded as an absorbing medium with a centro symmetric absorption spectrum function, whose central wavelength at absorbing peak has a red-shift as increasing the refractive index of measured medium. The features of this centro symmetric absorption spectrum function make it possible to overcome the negative effect on measurement sensitivity originating from zero-value point of differential spectra at absorbing peak in directly measuring spectral method by the application of frequency modulation principle. And the adverse effects, rising from the intensity fluctuation of light source and in light propagating paths, can be suppressed by introduction of reference light path and negative feedback closed-loop system structure. The probes, which are made up of 15-mm long silica multi-mode optical fiber plated with 50-nm thick gold film, are applied in the proposed model, the corresponding experiments are arranged to measuring different liquid mediums with the index range of 1.33000~1.43000 refractive index unit (RIU).Then, discussion and analysis are carried out based on the data, which are got from the proposed method and other methods, such as Abbe refractometer, optical fiber spectrometer and numerical emulation. The results indicate that the precision of the proposed model is 0.00016 RIU at the range of 1.33200~1.37580 RIU, which is the same as measured results by applying optical fiber spectrometer with 0.4 nm wavelength resolution, and the precision is 0.000710 RIU at the range which is better than measured results by using optical fiber spectrometer with 0.0009 RIU. Therefore, the feasibility and reliability of the proposed method and the application for on-line measurement can be confirmed.