In the case of planar structure such as large-scale rough surface, most time consuming of the iterative Robin boundary condition (IRBC) evaluation is spent on the planar fictitious boundary. To reduce the computation complexity of the IRBC, a field integral equation is written as the 2-dimensional convolution form and gets solved efficiently by using the fast Fourier transform (FFT). The formulation of its mechanism is presented. And a simulation with finite element method of the electromagnetic scattering from 3-dimensional random rough surface is shown for the accuracy and efficiency.