The Ga K fluorescence X-rays emitted from the incident surface of GaAs perfect crystal were observed around the (200) reflection peak in the Laue case,while tuning the energies of incident X-rays from a synchrotron radiation source between the K absorption edges of the Ga and As atoms.With the change of the energies of incident X-rays,the asymmetry of the fluorescence X-ray curves will change accordingly,the tendency is similar to that in the Bragg case.However,the variation in the Laue case cannot be explained by the moving of the nodal planes of X-ray standing wave (XSW) with respect to (200) diffraction plane of GaAs.In the Laue case,the variation of the amplitudes of XSW with the incident angle is closely related to the phase of the structure factor,whose change will result in the change of the asymmetry of fluorescence curves.Under the assumption that the fluorescence yield is proportional to the electric field intensity at the position where the atoms are located,the formulae for calculating the fluorescence emission in the Laue case for perfect crystals are derived.The agreement of the experimental results with the new formulae is quiet good.