Abstract: It is founded that the prompt neutron decay characteristics of the metal reflection layer subcritical device and prompt neutron decay characteristics of the hydrogen reflection subcritical device are not the same. For the fast spectrum device, after the DPF neutron source is induced into the device, the energy spectrum of the device will be basically unchanged, the time constant of the eigenvalue calculation concept and numerical eigenvalue can be applied to research the fast spectrum device, and the numerical calculation of time constant eigenvalues will be same with the experimental measurements for comparison. However, for the hydrogen reflective layer device, the energy spectrum will changed till reaching the steady state of the device, based on the definition of eigenvalue, the energy spectrum of the hydrogen reflector device reached steady state within 10 milliseconds or longer, the longer the more information can slow neutron the spectrum, so the eigenvalue is not applicable for hydrogen reflector device.