Abstract: Experiments were performed for determing prompt neutron decay constant α of highly enriched uranium spherical shell by randomly pulsed neutron nethod with a ²⁵²Cf source. α values from both least squares analysis of single exponential and two exponential fittings are in agreement and it is 100μs^-1. α value calculated of 110μs^-1 is also obtained by Monte-Carlo method. It is proved that the time-to-amplitude converter measuring system is effective for α determination of deep subcritical assemblies.