Abstract: A method combining the neutron transportation and the computationalmethod of neutron coincidence probability related to time is presented. This method isused to perform numeral simulation for detecting a hypothetical nuclear warhead fissionevents induced by external low energy neutron source. The feasibility of verifying nu-clear warhead by active coincidence neutron detecting is studied. The results show that,using external neutron source with energy less than 1 MeV, the coincidence neutronscounting rate is one order of magnitude higher in the model of nuclear warhead than thatof a fake nuclear warhead, which has the same configuration but with natural uraniumcore replacing high enriched uranium core. Therefore, the method of coincidence detec-tion could be used in authenticating the attribution of nuclear warhead.