Abstract: Rossi- measurement is commonly used to determine the prompt neutron decay constant. The relation between prompt neutron decay constant and Rossi- distribution are established on the condition of weak neutron source. When the source intensity is high, the measurment could not provide correct result, due to the overlap of neutrons emitted from different sources. In order to quantitatively analyze the influence of neutron source intensity on Rossi- measurement, simulation tools to calculate the Rossi- distribution are developed on the basis of MCNP code. In this paper, two sketch models of subcritical systems, a reactor model and an accelerator driven system model, are simulated to illustrate the relation between the estimated prompt neutron decay constant and the source intensity. Simulation results indicate that when the neutron source has high intensity, the calculated prompt neutron constant will deviate from the actual results. For both subcritical reactor and accelerator driven system models, when the intensity of the neutron source is low, the prompt neutron decay constant could be obtained by Rossi- measurement, while, when the intensity is high, the measurement could not provide correct result of the constant. By studying the interrelationships of the source intensities and the measured results, the maximum usable intensity value which could provide correct measurement result could be obtained. It is possible to reduce the detection duration by choosing a relatively stronger usable neutron source within the range of the maximum usable intensity.