Room temperature photoreflectances of a MBE GaAs doping superlattice were measured under different pump beam intensities. A series of fine structures corresponding to forbidden transitions (△n≠0) were observed af lower pump beam intensities. This means it is net a third derivative spectrum. A first derivative lineshape of photoreflectance for doping super-lattices was suggested, and the modulation mechnism was discussed. Theorefical calculations give a good explanation to the experiments.