Abstract: In order to study impact of the core assembly installation on flow rate distribution among core assemblies, three-dimensional numerical simulation was carried out with CFD code FLUENT to investigate the flow rate distribution of 7 assemblies installed on China Experimental Fast Reactor (CEFR) type-1 distribution header. The calculation results show that the relative deviations of calculated flow rates and their design values under 22 kinds of assembly arrangements are 0.41%-2.03%. Not only the flow distribution in the hottest tube can be more accurately determined, but also the reference for the assembly installation in CEFR is figured out according to simulation results.