A new method is proposed to rapid compute the electromagnetic scattering from the large object on ocean surface. This method combines the Geometric Optic (GO), the Physic Optic (PO), the Shooting and Bouncing Ray (SBR) and the Method of Equivalent Currents (MEC). The interaction between object and ocean surface is computed by using GO/PO+SBR, and the result is improved by MEC which takes into account the edges diffraction. The shadowing effect is also included. The radar cross section (RCS) of regular metal object located on ocean surface is computed by using this method. The validity of this method is proved by comparing with the traditional method of moment (MoM). Finally, a aircraft carrier object located on random ocean rough surface with PM(Pierson-Moskowitz)spectrum is computed, The result is analyzed, and the influence on scattering characteristic caused by parameters of ocean surface and incidence wave is discussed.