Abstract: Acoustic scattering from an object near an interface is studied by use of the physical acoustics (kirchhoff approximation) method. An integral representation is derived for calculation of the form function or target strength. The resulting scattered field consists of three parts: (1) the scattered field from the object in full space; (2) the scattered field, in which either the incident wave or the scattered wave is reflected by the interface, just as the bistatic scattered field; (3) the scattered field from the image object, in which both the incident and scattered wave are reflected by the illterface. For the situation of a rigid sphere near a flat free surface, a numerical comparison shows the present method agrees well with an exact analytic method. the present method provides a simple evaluation to echoes from objects submerged under sea surface or laid on sea bed.