Abstract: The classical Kirchhoff formula in the iterative physical optics (IPO) approach, which was brought forward by Obelleiro(1995), has been replaced by a Kirchhoff formula with line integral form. Based on the new method, an improved IPO approach has been presented. The numerical precision of computation results obtained by the improved IPO approach is almost the same as that of the results computed by IPO approach in Obelleiro s paper(1995). However, the new approach reduces the computation time greatly in analyzing the electroniag- netic scattering of large open-ended concave cavities such as a trihedral corner reflector and an aircraft cockpit, etc.