Based on the approximative theory of Marcatili, an approximate mode expression of rectangular optical waveguide Ey00 mode is presented. Based on the angular spectrum principle of plane wave, the expression of diffraction spatial frequency spectra for rectangular optical waveguide Ey00 mode is deduced. The distribution characteristics of diffraction spatial frequency spectra for rectangular optical waveguide Ey00 mode in x axis is discussed. The beam propagation factors M2 of rectangular optical waveguide Ey00 mode in x and y axis are analysed. The spatial frequency half width of rectangular optical waveguide Ey00 mode is numerically analysed by power in the quadrate bucket, and the curve of rectangular optical waveguide Ey00 mode spatial frequency half width defined by power in the quadrate bucket versus normalized frequency is obtained.