The GaAs/GaAlAs heterojunctions with different thicknesses of cap layer were investigated by the use of photoreflectance (PR) spectroscopy. By analyzing the Franz-Keldysh Oscillation (FKO) in PR spectrum, it was found that the thickness of cap layer would influence the surface band structure of heterojunction, and the surface field was increased with the decrease of thickness of cap layer. It was also found that the PR lineshape would rotate with the varying of the thickness of cap layer. This is coincident with the theoretical calculation while considering the interference effect between different layers.