Abstract: Flow under low Reynolds number condition like laminar and transition flows is very important to the safety operation of the fast reactor which can exist in the different kinds of assemblies under normal operating and accident shutdown conditions. The hydraulic characteristics of different kinds of wire-wrapped rod bundle assemblies of China Experimental Fast Reactor (CEFR) under low Reynolds number were analyzed by using CFX program. The results show that the calculation result of hydraulic characteristic under low Reynolds number is in good agreement with the experimental result and the Engel’s relational expression by using one helix wire-wrapped rod bundle assemblies as the calculating models. Horizontal flow produced by the wire-wrapper can induce the distinction of pressure distribution on six walls of the assembly, according to the calculation result of four helixes wire-wrapped rod bundle assemblies, but the pressure loss along the axial direction of each face is uniformly distributed by helix when the flow is fully developed. During measuring of the hydraulic characteristic of wire-wrapped rod bundle assemblies, the measuring points need to be placed on the same wall and the distances from inlet are multiples of the helix which can avoid the influence of the measuring caused by the horizontal flow.