Surface Plasmon resonance (SPR) technique is gradually growing as a research focus in optical area, which has been widely applied in many disciplines. The fiber-based SPR sensor, combining the advantages of fiber and SPR phenomenon, will lead to more tremendous application areas. The design and fabrication of different fiber SPR sensors are always be focuses of researchers in recent year, but the characteristic wavelength demodulation of the SPR sensor didn’t obtain enough attention. In this paper, the detection system for a fiber-based SPR sensor was presented first, and the detection mechanism of the refractive index sensing was obtained theoretically. Based on the theoretical simulation and analysis, the analytical expression of the all-phase low-pass filters was deduced, which is used for the de-noising of the original spectrum for the SPR sensing system, leading to resonance wavelength extraction only by calculating the derivative of the de-noised signal. So the resonance wavelength of the SPR sensor can be easily obtained by using this simple demodulation algorithm. Based on the realization of the system, the refractive index sensing of alcohol solution was carried out. And a good linear relationship between the refractive index of the solution and the characteristic wavelength of the SPR sensor can be obtained, with refractive index sensitivity and fitting error of 2048.5 nm/RIU and 5.69 nm respectively.