In order to study of the effect of fiber optic surface plasmon resonance (SPR) sensor parameters on refractive index measurement sensitivity, refractive index measurement experiments were carried out by using the dual-frequency laser heterodyne interferometric phase measurement optical path combined with the optical fiber SPR sensor. The effect of different core diameters on sensor sensitivity of fiber optic SPR sensors was theoretically analyzed. Within the refractive index range adapted by fiber optic SPR sensors, optical fibers with a core diameter of 300 μm and fibers with a core diameter of 400 μm were used to measure the phase difference of glycerol, sucrose and sodium chloride solutions under different mass fractions, and to calculate the corresponding refractive index. The relationship between the mass fraction and refractive index of each solution within the refractive index range adapted by the sensor was analyzed, and the theoretical results were experimentally verified. The results show that the smaller the diameter of the core, the higher the sensitivity of the sensor, and the sensitivity can reach 10-5 orders of magnitude; The higher the density, the higher the stability in the measurement, and the maximum phase difference standard deviation is 0.145°; The larger the molecular weight, the higher the accuracy, and the difference between the measured calculation of sucrose and the calibration value of the Abbe refractometer is up to 0.52×10-4. This research provides a good foundation for the further research and application of optical fiber SPR sensing technology.