Gaussian beam has been widely employed in optical particle measurements recently, in which the light propagation and the beam′s parameters are very important to measurement results. An analytical expression of diffracted field of a Gaussian beam through a planar interface is derived based on Fresnel-Kirchhoff diffraction integral and under paraxial approximation, which is checked with numerical calculation. Result shows that, while a Gaussian beam, which is weakly focused, propagates normally through a planar interface between materials of mismatched refractive indices, parameters of beam waist radius keep unchanged and location of the beam waist depends on the ratio of refractive indices of neighboring materials.