Based on the characteristic of the beam propagation factor of the diffractive beam from the end surface of dielectric planar waveguide, the rationality of the Gaussian approximation for the mode field distribution of planar waveguide TE0 mode is clarified. As the computing formula of the matching efficiency between two field distributions is engaged, the method of equivalent matching efficiency is advanced for ascertaining the equivalent mode field half width of the Gaussian approximation expression, and the computing formula of equivalent mode field half width which is expressed by the half thickness of core layer, the normalized standing wave parameter of core layer and the normalized evanescent wave parameter of cladding layer of the planar waveguide are acquired, and the matching efficiency between the Gaussian distribution and the eigen mode field distribution of planar waveguide TE0 mode is presented. The rationality of the method of equivalent matching efficiency for computing equivalent mode field half width is explained. Finally, as the method of solving the equations is introduced, the fitting expression of equivalent mode field half width which is expressed by the half thickness of core layer and the normalized frequency of planar waveguide is suggested, and the accuracy of fitting expression is showed by the error analysis.