Abstract: To evaluate the influence of leaking neutron spectrum and neutron flying time to the measurement of prompt neutron decay, prompt neutron decay curves at different locations inside and outside a polythene reflected plutonium metal sphere were calculated with MCNP-4C. Calculation results shows that the flying time difference for leaking neutrons with different energies distorts the curves by slowing down the decay and with the increase of the flying distance the distortion becomes more severe. When the flying time difference of the detected leaking neutrons is comparable to the time decay constant of the nuclear system, the time decay rate of leaking neutrons can not characterize the prompt neutron decay of the nuclear system.