This paper reports the photoreflectance(PR) measurements of electric fields at the surface and the GaAs-GaAs interface of doped MBE GaAs films. By using alternatively He-Ne laser and He-Cd laser as pumping light, PR signals from the surface and the interface of the film can be effectively seperated the different penetration depth of different light. Electric fields at the surface and the interface were deduced from PR spectra. Film interference effects on modulation spectroscopy are studied. The origin of the electric field generated at interfaces are also discussed.