Aiming at the urgent requirements for refractive index detection in the biological sample detection area, an all-fiber surface plasmon resonance (SPR) system is established in this paper. And the SPR characteristic wavelength demodulation algorithm is proposed for this system based on all-phase filter technique. According to the system simulation, the refractive index sensing sensitivity of the fiber SPR sensor can be calculated theoretically. By using the all-phase filter technique, the characteristic wavelength of the fiber SPR sensor can be extracted, and the theoretically analytical expression of the all-phase filter can be obtained. The experimental results show that the refractive index sensing sensitivity and the detection resolution of the fiber SPR sensor are 1640.4 nm/RIU and 7.3610-4 RIU respectively by using this algorithm. Compared with the traditional methods, our algorithm can improve the detection precision and the anti-light-disturbance performance and reduce the costs as well.