Optical fiber surface plasmon resonance (SPR) sensor has some challenges for its wide applications, such as difficulties of fabrication, complexities of sensitivity enhancement methods, and high costs. To solve these problems, a highly sensitive SPR biosensor was fabricated in this study. A commercial multimode optical fiber was used with a conventional coating method. The fiber was pretreated by physical method, and the gold nanoparticle film was plated vertically in the ion sputtering instrument. Following these processes, the optimization of coating parameters was conducted by using the spectral shift sensitivity of the relative refractive index of the solution as the index. Then, an SPR sensing system was developed with this new biosensor, tungsten lamp light source, Y-type optical fiber, spectrometer and spectral test and analysis software. The results demonstrated that the coating current and coating time were 15 mA and 20 s respectively. Characterization of the sensor's structure of this biosensor with scanning electron microscope (SEM) showed that a layer of non-uniform gold nanoparticle film with a gradual decrease in axial particle size from the front end to the tail end was sputtered on the surface of the fiber. The sensitivity of the sensor can reach 3416 nm/RIU in the refractive index range of 1.339~1.384 with different concentrations of glycerol aqueous solution as samples. On this basis, the optical fiber SPR sensor was carboxylated followed by cross-linking with Protein A ligand and an SPR biosensor was prepared for HIgG immunoassay. The results showed that the linear correlation coefficient R2 of the sensor for HIgG immunoassay in the range of 1.25~20.00 μg/mL was 0.9987, and the sensitivity and detection limit were 0.6786 nm/(μg/mL) and 0.15 μg/mL, respectively, which were comparable to those of sensors reported in the literature that underwent complex sensitization processes. In summary, the novel SPR biosensor has the advantages of simple fabrication process, stable performance and high sensitivity, which has a wide range of application prospects.