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A cascaded side-polished multimode fiber sensor based on surface plasmon resonance (SPR) is proposed. The sensing area is a multimode optical fiber in which half of the core has been polished away and a 40-nm gold thin film is deposited on to the polished surface. The cascaded structure is demonstrated in the small difference variation of the refractive index between the two solutions, deionized (DI) water and phosphate buffered saline (PBS). The transmitted intensity of SPR fiber sensor in dynamic measurement between single SPR fiber structure and the cascaded structure achieved to increase the variation from 73% to 58% for DI water and 70.5% to 53% for PBS, and enhance the difference between DI water and PBS from 2.5% to 5% is obtained in the experiment. The stability and the double variation to enhance the high sensitivity of the intensity measurement is achieved to be 2 times 10-5 refractive-index units.