Abstract: The subcritical system driven by external neutron source is prevalent in nuclear engineering. Due to inherent ray effect and spatially localized source, it is difficult to accurately compute the flux density in this system by the discrete ordinates (S_N) method. Although the Monte Carlo (MC) method is suited to simulate localized source problems, it always leads to the poor computational efficiency. Thus, accuracy and efficiency can not be met simultaneously when only one method is adopted. To take advantage of both methods, a coupled MC/S_N method by the first fission source was introduced. First, neutrons were tracked from the source to their first fission, and the first fission source was tallied. Next, the transport equation with the first fission source was solved by S_N method. Finally, the required flux density was equal to the sum of the flux densities calculated by MC and S_N Numerical results indicate that the coupled method is efficient and accurate.