When copper vapour laser mirror is used at an abnormal incidence the different phase shift of the polarization states may occurred. An optimum technique has been used to determine the layer thickness for a coating design that produce a 90° phase shift between the p- and s-polarization components over 490～530 nm while the average reflectivity is above 99.998%. The thickness of Ag layer is insensitive to the phase shift. There is no effect on the transmittance when the thickness is larger than a constant. A tolerance analysis indicates that the deposition rate should be controlled within ±1% to achieve ±15.28° phase shift error, with a convergence of the phase shift about 504 nm. The phase shift error could reach ±12.77° within ±1% of the refractive index variation over the designed wavelength range. The variation of the outmost layer's thickness also is controlled within ±1% to achieve ±5.5°, and the phase shift error caused by the layers from 2 to 5 and 9 to 16 is above ±0.5°, but the other film layers have little influence on the phase shift. The incidence angle also is controlled within ±1% to achieve ±2.86° phase shift error over the designed wavelength range, the waveband around 530 nm is insensitive to the incident angle.