Based on the theory of multimode interference and self-imaging effect, a high sensitivity multimode interference-heterocoreless (SNS) fiber optic refractive index sensor was designed. The multimode interference is generated by the core mismatch between the cladding excited high order mode and the fundamental mode coupling in the coreless fiber to realize the sensing measurement of refractive index. The beam propagation method (BPM) was used to simulate the transmission spectrum of the sensor under different refractive index conditions, and the influence of the length of the coreless fiber and the refractive index of the external environment on the performance of the sensor was discussed. The transmission spectra of sucrose solution with different concentrations were measured by the preparation of a coreless fiber SNS structure sensor, and the experimental results were consistent with the numerical simulation results. The results show that, in the range of refractive index of 1.330~1.419, the wavelength sensitivity of transmission valley reaches 189nm/RIU, and its transmittance sensitivity reaches -40%/RIU.