The waveplates are used in the laser technology and the polarization microscope often in oblique incident beam. The accurate formulas of phase shift and polarization aberration are presented. The formulas are very important calculating accurately the tuning curve of waveplates and polarization aberration of polarization microscope. The calculations show that if the azimuth angle of crystal is selected properly, the phase shift of waveplates and the ellipticity of polarized light change with incident angle can be greatly decreased.