In this paper

we use OptiGrating software to investigate the spectral characteristics of TFBG different modes when surrounding refractive index changes extremely little.Result shows that the micro change of refractive index has very little impact on core mode and phantom mode

but has great impact on the cladding modes.Because the change is very little

the drift of cladding mode resonant wavelength can be neglected

and only the variation of the resonant peak depth needs to be considered.The high-order cladding modes near 1 542 nm wavelength are sensitive to the surrounding refractive index.In a certain range of refractive index

the precision is extremely high with orders of magnitude up to 10-5.The simulation results obtained in this paper has broad prospects in high precision refractive index sensing field.