The reflected spectra of GaN-based DBR at different incident medium are studied theoretically by transfer matrix method. The results show that the reflectivity of DBR depends on the refractive index of the incident medium with quadratic function when the periods of GaN-Al_xGa_1－xN DBR are under 15 and the reflectivity is about linear with the refractive index of the incident medium when the periods are over 15.Based on this relation,a simple formula is deduced in order to estimate the actual spectral reflectivity of DBR in LED. The differences of the reflected spectra of different GaN-based DBR at air and Al_0.4Ga_0.5In_0.1N incident medium are analyzed in detail. A half-hybrid GaN-based DBR structure designed is more predominant than conventional DBR structure designed in reducing the difference of the reflectance spectra of DBR at different incident medium,improving material growth and enhancing light extraction of LED.