A comprehensive facet model for bistatic synthetic aperture radar (Bis-SAR) imagery of dynamic ocean scene is presented in this paper. An efficient facet scattering model is developed to calculate the radar cross section (RCS) of the ocean surface for Bis-SAR firstly. Further more, this facet model is combined with a bistatic velocity bunching ($VB$) modulation of long ocean waves to obtain the Bis-SAR intensity expression in image plane of ocean scene. The displacement of the scatter elements in the image plane and the degradation of radar resolution in azimuth direction are quantificationally analyzed. Finally, Bis-SAR imagery simulations of ocean surface are illustrated, proving the validity and practicability of the presented algorithms.