Abstract: The ultra fast neutron detector is the key device in ICF fusion reactionrate measure system. The performance of the ultra fast neutron scintillator BC-422 was simulated using Geant4 code system. The detection efficiency, temporal resolution and signal intensity of BC-422 with different thickness were calculated. The temporal resolution and signal intensity of BC-422 cladded with two kinds of reflection surfaces were compared. The results show that the properly designed BC-422 scintillator can perform well above 108 neutron yield, and its temporal resolution is better than 20 ps for DT fusion neutron, and 30 ps for DD fusion neutron. The BC-422 scintillator is applicable to the fusion reaction-rate measure system for a large-scale laser facility and its prototype requiring tens of ps-scal