Highly enriched uranium subcritical assemblies with and without low atomic number (low-Z) reflectors are modeled. The neutron and photon transport process in the assemblies initiated by a pulsed neutron source is simulated using the Monte Carlo code. When the size or mass density of the fissile region and the reflector changes, the effective multiplication factor k_eff of the fissile region or the whole system changes accordingly (for assemblies without reflectors, the fissile region refers to the whole system). For each case, the time dependence of leakage γ count rate, neutron population in the fissile region and their ratio is obtained and analyzed to investigate the applicability and limitation of the pulsed neutron source (PNS) method for subcritical systems with and without reflectors. It is indicated that the PNS method is suitable for systems of which the fissile region is near-critical, i.e., the k_eff value of the fissile region is close to 1. The closer the fissile region is to the critical state, the more accurately the leakage γ count rate can reflect the fissile region's fission decay properties during the specific time window. For the highly enriched uranium assemblies described in this paper, the k_eff threshold of the fissile region ensuring the applicability of the PNS method is given. In addition, the possibility of applying the PNS method under dynamic conditions are also discussed according to the calculation results.