Abstract: Use Huygens principle the extraordinary light total reflection angle formula of negative uniaxial crystal for the optical axis in incidence plane i = arcsin 1/n√（no^2-nc^2）cos^2a＋n^c2is deduced, and this formula is applied to design and improve Glan-Thompson polarizing prism which is the most economical in consumption of materals as the optical axis is vertical to the interface and θ=26°.

Key words: Huygens principle, negative uniaxial crystal, extraordinary light total reflection, Glan-Thompson polarizing prism