Abstract: In order to effectively solve the shielding problem with complex geometric and bulk shields of nuclear facility, the threedimensional Monte Carlo (MC)-discrete ordinates (S_N ) bidirectional coupled method was developed. The transform between particle probability distribution of MC and angle flux density of S_N was realized by a self-developed interface program to achieve the coupling calculation. The test calculation was taken by the application of the MC-S_N bidirectional coupled method on the particle fluence rate calculation of reactor pit bottom. Monte Carlo code was employed to accurately simulate the structure of core and the model of reactor pit, while the 3D S_N code was used for the calculation from the core bottom surface to the pressure vessel bottom region. Neutron and photon fluence rates of reactor pit channel and indoor were calculated by MC-S_N-MC two step coupled method. The calculation results were compared with results of MCNP and satisfactory agreements were obtained. The results demonstrate that the method is an effective tool to solve the shielding problem of large complex shielding of nuclear facility.