Abstract: In the study of the method to diagnose the prompt neutron decay constant of the subcritical device by the Dense Plasma Focus source, the different neutron time characteristics are found in the subcritical device with different reflective layer, such as a metal reflector and a hydrogenous reflector. The different neutron time characteristics like k_eff, prompt neutron generation time, energy spectrum, and the different of prompt neutron decay constant within different energy segments and different geometric segments is analyzed by the Monte Carlo method. The results showed that, in the subcritical device with a hydrogenous reflective layer, within 10-6 s time scale, the leakage fast signal of the device can fully reflects the characteristics of the active region. In 10-6 ~10-4 s or much longer time, the shape of the leakage neutron spectrum and space spectrum may fully reflects the whole device.