This paper presents a hybrid iterative algorithm of analytic KA-numerical MOM for scattering computation from a 3-dimensional (3-D) perfect conducting target above a randomly rough surface. The coupled integral equations (IEs) are derived for difference scattering computation. The method of moment (MoM) with the conjugate gradient (CG) approach is used to solve the target's IE，and the Kirchhoff approximation (KA) is applied to scattering from the rough surface. The coupling iteration takes account of the interactions between the target and the underlying rough surface. The convergence of this hybrid algorithm of KA-MoM is numerically validated. Since there is only one numerical integral along the rough surface performed for KA computation，memory and CPU time are significantly reduced. Numerical results of bistatic scattering from a PEC ellipsoid or cubic target above a Gaussian rough surface produced by Monte Carlo method are obtained.