We theoretically simulate the influence of the scattering processes of the electrons in the continuum states on the shapes of the photocurrent spectra of THz quantum well photodetectors.The width of the photocurrent peakshould be wider according to the uncertainty relation when these scattering processes lead to shorter relaxation time.We take a simple approximation model to include this influence and calculate the photocurrent spectra,which axe in agreement with experimental results qualitatively.