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• GaS nanosheets used for the first time as a sensitizer material in optical fiber SPR sensors is proposed. • GaS modification of optical fiber is achieved through layer-by-layer self-assembly. • The experimental results show that the SPR sensor based on GaS nanosheets offers high sensitivity and FOM value. Optical fiber Surface plasmon resonance (SPR) sensors are applied in the field of biological monitoring widely. This paper presents a novel method to improve the sensitivity of optical fiber SPR sensor by using gallium sulfide (GaS) nanosheets. Compared with traditional two-dimensional materials, gallium sulfide has a higher refractive index and higher carrier mobility, which can effectively enhance the surface plasma wave (SPW), thereby improving the sensitivity. By testing sensors coated with gallium sulfide of different thicknesses, the team finally found that when the sensor structure is set to MSM, the thickness of gold film and gallium sulfide nanosheets dispersion is 50 nm and 40 nm, the sensitivity and the FOM value of SPR fiber sensor are 5858.78 nm/RIU and 36.62 RIU−1. The sensitivity to refractive index is 2.9 times that of the traditional gold film sensor, and the sensor has good stability and reliability, and can be reused. This sensor can be applied to monitor low concentrations of biomolecules and weak signals. The use of this material has great implications for the production of new SPR-based chemical and biosensing devices. [ABSTRACT FROM AUTHOR]