Abstract: he Jones matrix of birefringent crystals is derived according to the separate transmitting theory of the two eigen polarization states of a birefringent crystal. The Jone's matrix that the eigen optical wave is line, circle or ellipse is analyzed and discussed. The direct factors affecting polarization state while light goes through a crystal is found and their relation curves are drawn. The opticity of the crystal and its change is analysed. It has a directive function for the installation of crystal in differential laser gyro.