We have deduced a formula for sensitivity of reflection spectrum from the general relation between dielectric constant and reflectivity. Using this unified formula, we have analyzed the sensitivity of reflection spectra of Lorentz oscillator and Drude free carrier, thus we have got a deeper understanding of the abnormal reflection-spectrum structures on the short-wave side of reststrahlen bands of some alkali-halide crystals and the feature of infrared reflection-spectrum of high-Tc superconductors.