We have studied the optical properties of the interface of Al and GaAs surface q uant um well by insitu photoreflectance (PR) spectroscopy in a molecular beam e pitax y (MBE) system. The intermixing of Al and GaAs surface quantum wells forms an AlxGa1-xAs barrier layer on GaAs, which would shift the inte r_band transition peaks of the GaAs quantum well. Based on the calculation using effective mass approximate method, we fi nd that the intermixing length is 0.5nm, which is an important parameter in sem iconductor technology.