The characteristics of resonant guided-mode is analyzed under transverse magnetic (TM) polarization light at the Brewster angle using the rigorous coupled-wave analysis (RCWA) and effective media theory (EMT) and the effect of different structure parameters on label-free detection sensitivity is analyzed. The effect of three film structures on detection sensitivity is discussed. It is found that the shifts of resonant angles are 5.1°, 3.6°and 4.2° for single-layer resonant Brewster sensors (RBS), double-layer RBS and triple-layer RBS when the refractive index changes by 0.1, and all show high dectection sensitivities. As the modulation index reduces from 0.4525 to 0.1441 for single-layer RBS, the refractive index changes by 0.1, the changes of incident angles are 4.48° and 4.95°, and the detection sensitivity increases.