The electromagnetic (EM) scattering computation and the synthetic aperture radar (SAR) imaging of three-dimensional conductor object located on ocean surface are studied. The EM scatterings of object, ocean surface and the interaction between them are computed based on the geometric optic, the physical optic, the shooting and bouncing ray. The method of equivalent current is used to calculate the diffraction by object edges. The shadowing effect is also included, and the tapered incident wave is chosen to reduce the truncation error. The Pierson-Moskowitz random sea surface is generated by using Monte-Carlo method, and numerical results are provided to validate the approach through the computation of radar cross section for cube and ship objects located on ocean surface. The backscattering electric field data of different frequencies and angles are computed quickly by using this approximate method. Combining the backscattering data and SAR images technology, the images of cube and ship located on ocean surface are obtained. The method introduced in this paper has important theoretical significance in realistic ocean remote sensing and detection of military targets located on ocean surface.