We propose a modification of the interface roughness (IFR) scattering form factor in intersubband transitions.We properly derived a formula for the form factor for IFR scattering in terms of the integrals of the envelope wave functions. This novel form factor is more global nature than the old one (proposed by Ando) and may be suitable for a wide range of applications. In this paper, we calculate and compare the absorption linewidth with the application of the old form factor and novel one. In different from previous calculations, with the same surface profile (Δ, Λ), the calculation results the interface roughness scattering absorption linewidth with the application of the new form factor is greater than twice the old one. Our numerical calculations may better explain the experimental results the well-width dependence of intersubband absorption linewidth.