An electromagnetic scattering solution for the interaction between the perfect conducting plate and the 1-D conducting slightly rough surface is presented in this paper. Taking the advantage of a newly developed technique that utilizes the reciprocity theorem, the difficulty in formulating the secondary scattered fields from the composite target reduces to the evaluation of integrals involving the polarization currents of the conducting plate and the scattered fields from the slightly rough surface. This method used in this paper can avoid the complex evaluation if scattering field under non-planar wave illustrated when calculating the secondary scattering fields. The polarization currents of the conducting plate is evaluated by using Physic Optics (PO) method, and the scattering fields from the slight rough surface are evaluated by using the SPM method. The results obtained have been analyzed in detail.