Central wavelength of S-polarization and P-polarization will separate when the filter is tilted .But the central wavelength of two polarization components in angle-tuned filter can be coincided by adjusting effective refractive index of spacer.Beginning with the analysis of phase relationship and by use of the characteristic matrix expression of multilayer films, two implicit function expressions of middle layer's refractive index of spacer and central wavelength of P-polarization and S-polarization are deduced. Therefore the value of middle layer's refractive index and corresponding central wavelength can be quickly and accurately obtained. Simulation calculations of two concrete filter designs for single cavity and three cavities prove that the method is feasible. Based on the samples data analysis of difference curves with incidence angle from 0° to 30°, it is proven that the algorithm is highly accurate.