Abstract: The randomly pulsed neutron method with ²⁵²Cf, which evolved from repeated pulsed-neutron method, is effective on prompt-neutron decay constant measurements. Because it’s based on time-correlated accordance, the influence of the background rays could be suppressed effectively. Using this method, subcritical measurements were performed on an alpha-phase plutonium sphere reflected by 31 mm thick and 29 mm thick steel. The proportion of useful signal and background was 1∶1 in the experiments, but in the prompt neutron decay spectrum the proportion of time-correlated signal and background was 7∶1. The prompt neutron decay constants of the measurements were 2.25 μs^-1 and 3.00 μs^-1 and the fitting error was ±0.02 μs^-1. Comparing with the results by Rossiαmethod, the difference is less than 1.3%.