According to the Rayleigh-Sommerfeld diffraction scalar integral formula,the equation of spatial frequency at half maximum intensity of central bright fringe of far field diffraction of planar waveguide in TE_0 mode was educed.It was also illustrated that the more is the value of normalized frequency of planar waveguide,the more is the value of the full spatial frequency width at half maximum.The relationship between the full angular width at half maximum intensity of central bright fringe versus wavelength,width of planar waveguide and relative refractive index of waveguide was presented too.