Abstract: It is necessary to shield the background neutron during the measurements of neutron decay constant on the zero power assembly.Based on the numerical calculation and experiment measurement,the radiation-shielding effects of the B_(4)C +(CH_(2))_n material to D-T neutron are gained.Using the results,detecting shielding system was designed for measuring the prompt neutron decay constant α of the zero-power assembly.The measurement result is agree with the design value.The signal-to-noise ratio(9∶1) of the neutron decay constant is similar to the numerical calculation(7.5∶1).