The photocurrent spectra characteristics of the 15μm very long wavelength GaAs/AlGaAs quantum well infrared photo detector(QWIP) have been investigated.The two photocurrent response peaks, which have a Gaussian lineshape due to the random fluctuation of the well width,are attributed to the transitions from the ground state E0 to the first excited state E１ and the third excited state E3 in the well,respectively.The transition peaks shift linearly to higher energy as the bias across the detector increases.Taking into account electrons exchange effect,we find that the theoretical calculation using the transfer matrix method agrees well with the experimental data.