Abstract: In order to enhance the gamma rejection capability of the GdI₃:Ce scintillator while improving neutron detection performance, the linear attenuation coefficient of gamma ray was calculated by Geant4 software and XCOM database. Both the simulation and the experimental tests were carried out to study the effectiveness of adding lead shielding layer for gamma rejection. The results indicate that the GdI₃:Ce scintillator is susceptible to the interference of low-energy gamma ray while detecting neutron. With the increase of lead thickness, the interference to neutron detection decreases for gamma ray in the energy range of 100 keV-1 MeV, and tends to increase firstly but decrease thereafter for gamma ray in the energy range of 3-10 MeV. Experimental results on the ²⁵²Cf neutron source show that the neutron peak comes to appear after adding the lead shielding layer. With the increase of lead thickness, the net count in neutron peak decreases but its ratio to the background count keeps increasing. It indicates that the thickness of lead shielding layer should be optimized according to the requirement for the neutron detection efficiency and the signal-to-noise ratio when the GdI₃:Ce scintillator detector is employed.