When incoming light beams enter dielectric reflective mirror with certain incident angle, there is phase shift between the p-polarization and s-polarization. Based on optimizing design, when light is incident at 45 degree, the phase shift of 270°±0.15° is got in the range of 1285～1345nm, at the same time, the reflectivity of dielectric mirror maintains over 99.5%. The phase retarder was prepared with dual ion beam sputter system and then annealed at different temperatures in atmosphere, the spectrum and the phase of the sample were gained by spectrometer and the Fourier transform infrared spectroscopic ellipsomter, respectively. The results revealed that the as-deposited samples gained phase shift of 267.5°±0.5° and the reflectivity reached over 99.6%. It was found from the fitting analysis that the error of the outermost layer and deviation between refractive index and designed values play principal roles in decrease of phase shift, in addition, with the annealing temperature increasing, the refractive index of film decreases and physical thickness increases so that phase shift drops off.