A total-field/scattered-field (TF/SF) technique is used to introduce an incident wave for a scattering problem in three-dimensional finite-difference frequency-domain (FDFD) method. According to the equivalence principle, an incident wave is introduced in total-field region by setting equivalent electromagnetic currents on TF/SF boundary. FDFD equations at nodes near TF/SF boundary are modified to satisfy conditions that all nodes involved belong either to total-field or to scattered-field. Numerical results validate this technique. The presented technique is mainly applied to scattering problem with a composite target,which is composed of perfectly electronic conductor (PEC) and lossy dielectric medium. Bistatic radar cross section (RCS) of a composite yon Karman missile warhead and a missile model are computed with FDFD method. The results demonstrate its applicability in analyzing electromagnetic scattering with composite targets.