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This article reports the spectral properties of a Ge2Sb2Se4Te1 (GSST) -coated tilted fiber Bragg grating to the changes of surrounding refractive index (SRI). The variations of the cladding mode resonance amplitudes and wavelengths under two orthogonal polarization modes are analyzed and validated with the simulation results. The introduction of the high index coating has induced non-degeneracy to the cladding modes, but the effect gradually diminishes with increasing SRI. The SRI sensitivities of the s-polarized modes have been substantially suppressed and became 20 times lower than that of p-polarized modes. The findings also reveal the association between the cladding resonance amplitudes, the imaginary part of the effective refractive indices and the evanescent field intensity distributions.