The optical field distribution in wide planar-waveguide CO₂ gas laser satisfies Helmholtz equation.Under certain given conditions,expressions of the polarized modes EH_nm^x and EH_nm^y are obtained by solving optical field distribution equation.The present computation gives a normalized theoretical distribution curve of EH_1.16 polarized mode existed in 3×18mm² waveguide discharge area.The curve is consistent with the experimental results.