In order to measure the change of the refractive index of solution, the three layers structure of fiber core-gold layer-liquid on the surface of plastic optical fiber core was studied.A surface plasmon resonance(SPR) sensor using U-shaped structure combined with side polishing method was developed. First, a segment of plastic fiber was bent into a U shape, and the curvature of the segment was fixed at higher temperatures in the oven. The outer surface of the curved part was then polished to expose the plastic fiber core layer at the position and to sputter the gold nano-film layer on it. Finally, the prepared sensor was immersed in the liquid with different refractive indices, and the movement of the spectrum was observed using a spectral analyzer. The experimental results show that when the plastic optical fiber SPR sensor measures the liquid with the refractive index variation range of 1.333~1.406, it can be observed through the spectrum that with the increase of the liquid refractive index, the plasma resonance absorption peak position is also moving towards the direction of wavelength increase, and there is a linear relationship between the two, and its sensitivity is 7.5×10-4 RIU/nm. The sensor probe has the characteristics of miniaturization, easy preparation and low cost.