The linear and nonlinear intersub-band absorption in GaAs asymmetric quantum wells (QWs) by difference frequency generation of two midinfrared waves are theoretically calculated in parabolic and nonparabolic QWs. The calculated results show that terahertz (THz) emission peak shifts in wavelength and THz emission coefficient is substantially reduced after considering band nonparabolicity. For the absorption coefficient of one of the two pump beams, it moves to lower energies by increasing its own wavelength and has little changes with the increase of the other wavelength. The leading causes of these phenomena are the changes of energy levels, the corresponding wave functions, and the transition matrix elements, which are caused by band nonparabolicity. Thus, it is necessary to make an accurate comparison of experimental intersub-band absorption with theoretical predictions.