Starting from the Schrodinger equation, the influence of different Al composition and different temperatures on the absorption spectra of wide quantum well infrared photodetectors (QWIP) is studied. It is found that when the Fermi level is fixed, the ground state bound energy of the quantum well may increase with the increasing of the Al composition and the corresponding absorption spectral peak tends to move toward the short wave. In addition, the influence of the ambient temperature on the response spectra of AlGaAs/GaAs QWIPs is negligible. After theoretical calculation, the rule of the absorption spectrum of an AlGaAs/GaAs QWIP changing with quantum well width, Al composition and temperature is given.