In this paper, the electromagnetic scattering from conductive target with open cavity has been simplified as two (inner and outer) equivalent models by using the equivalence principle and introducing an equivalent magnetic current in the aperture of the cavity. For the inner part of the cavity, the equivalent conductance matrix has been obtained by use of connection scheme. Approximate boundary in tegral method has been used to describe the coupling between inner and outer pa rts of the cavity, and to obtain the aperture equivalent magnetic current. Final ly, the general combined field integral equation was presented to form the elec tromagnetic model of the scattering problem, and the multilevel fast multipole a lgorithm was used to solve this problem efficiently. Numerical examples show go od agreement between calculated results and measured data.