Abstract: In the field of arms control verification techniques, the detection of the explosives in the nuclear warheads is an important research. In this paper, GEANT4 software was used to study the feasibility of the passive detection (using the fission neutrons in the nuclear warheads as the neutron source) of the explosives in the nuclear warheads with weapons-grade uranium cores (namely WgU warheads). The γ spectrum emitted from the nuclear warhead was calculated, agreeing with the expectation. However, due to the low event rate of the characteristic γ photons produced by the reactions of the fission neutrons in the WgU warheads and ¹⁴N in the explosive, this method is not applicable for the field measurement of the WgU warheads. So the feasibility of the active detection (using the neutron source outside the nuclear warheads to irradiate the warheads) of the explosives in the WgU warheads was studied based on GEANT4 software. The simulation results show that the ¹⁴N in explosives in the WgU warheads can be detected and confirmed within 1 h based on the active detection method, which can provide important evidence for judging whether the WgU warhead contains explosive.