A modified variational method is adopted to investigate the binding energies of the shallow impurity states near the interface of a strained GaN/AlxGa1-xN heterojunction by using a simplified coherent potential approximation. The relations between the impurity binding energies and pressure, the impurity position, electric field strength and the Al components are calculated for strained zinc-blende GaN/AlxGa1-xN heterojunctions with (001) and (111) orientations respectively. The result indicates that the binding energy of impurity state nearly linearly increases with pressure. The Stark effect on the binding energy as a function of electric field shows blue or red shift in the spectra for different impurity positions. The results corresponding to (001) and (111) orientations are compared. It is found that the effect on the binding energy of Al component is obvious. For an impurity located in the channel side and comparatively far from the interface, a monotonic increase of the binding energies will appear with increasing Al content due to the stronger two-dimensional properties of the electrons, and pressure enhances the increase of amplitude. Whereas for an impurity located in the barrier side, the increase of Al content will weaken the bound between the impurity and the electron to decrease the binding energy.