Based on the condition of paraxial approximation, the simple relational expression among the beam parameters, such as the mode field half width, far field divergence half angle and beam propagation factor M2, in the diffracted field of dielectric planar waveguide TE 0 mode is presented. The beam parameters can be calculated accurately by the mode field distribution of the waveguide, or by the diffraction frequency spectrum distribution in far field which is measurable exactly in the experiment. The calculational formulas of M2 factor are described by the ratio of mode field half widths defined by second moment and differential operator in near field, or the the ratio of far field divergence half angles defined by second moment and differential operator. Finally, the analytic function and scope of the beam parameters in the diffracted field of the dielectric planar waveguide TE 0 mode are demonstrated.