According to ISO 11146-1: 2005, mode-field half width, far-field divergence half angle and beam propagation factor of the diffractive beam from TE0 mode in planar waveguide are analyzed by a new method. Under the condition of paraxial approximation, formulas of diffractive beam parameters as function of waveguide parameters and slit half width are derived respectively. Relation between beam parameters and slit half width are investigated by numerical calculation. The results show that beam parameters vary with changing slit half width, and if the optical field distribution in the cladding layers is not taken into account, beam propagation factor will be less than unity.