This paper presents a novel and powerful numerical technique called general sparse-matrix flat-surface iterative approach-canonical grid (G-SMFSIA/CAG), which combines the sparse-matrix flat-surface iterative approach-canonical grid (SMFSIA/CAG) for 2D conducting rough surface and the method of moment (MoM) based on Rao-Wilton-Glisson (RWG) function for PEC target, to compute the electromagnetic scattering from a three-dimensional PEC target on conducting rough surface. The coupling surface integral equations (SIEs) for the composite model are derived based on Huygens surface equivalence principle. The SMFSIA/CAG is applied to solve the SIE of rough surface, and the targets’ SIE is solved using MoM. The iteration of SMFSIA/CAG and MoM takes account the interactions between target and the rough surface. Combining the PM(Pierson-Moskowitz)sea surface generated using Monte-Carlo method, the bistatic scattering coefficient for different targets on sea surface are computed. Convergence and effectivity of the G-SMFSIA/CAG is numerically validated. The bistatic scattering coefficient from ship on sea surface is calculated finally,and the dependence of bistatic scattering pattern upon the wind speed is discussed.