The temperature-and voltage bias dependent photocurrent spectra of very long wavelength GaAs/AlGaAs quantum well infrared photodetectors were studied by spectroscopic measurements and the corresponding theoretical calculations. It is found that the peak response wavelength will shift with the changing of voltage bias and temperature. With the assistance of band structure calculations, we proposed a model of the double excited states which explains the experimental observations very well. Meanwhile, the working mechanism of the quasi-bound state confined in the quantum well, including the processes of tunneling and thermionic emission, were also investigated in detail. Based on our model, two transition processes, including the ground state to the first excited state transition and the ground state to the continuous state transition have been separated from the photocurrent spectrum. The two normalized photocurrent spectra peak wavelength agreed reasonably well with the calculating results. The results allow a better understanding of bound-to-quasibound state transition and bound-to-quasi-continuous state transition and thus a better optimization of the QWIP performance.