Photoreflectance (PR) spectroscopy was used to analyze the GaAs/AlGaAs multiple quantum well (MQW) infrared detector structures. The result shows that the important parameters such as the well width, the aluminum composition x, the energy of intersubband transitions and peak wavelength of the detector can be exactly deterllilned by PR spectroscopy. The calculated energies of intersubband transition based on the Kronig-Penny model were compared and proved to be in agreement with the experimental results.