As a new fuel assembly design scheme, the Horizontal Honeycomb Fuel (HHF) assembly has some advantages in the energy supply of underwater equipment. HHF assembly can meet the needs of long life and lightweight reactor core. In this paper, a fuel assembly of 37 coolant pipelines was numerically simulated based on CFD method. The distribution characteristics of coolant temperature and flow velocity in the HHF assembly were obtained. The results show that under the influence of gravity, the coolant temperature and flow velocity in the circular tube no longer show a center-symmetric distribution. The minimum temperature point and the maximum flow velocity point of the coolant appear below the center point of the pipeline, and gradually move away from the center point with the increase of axial distance. This research reveals the flow and heat transfer characteristics of liquid lead–bismuth eutectic coolant in horizontal honeycomb fuel assembly to a certain extent, and provides a reference for the design and optimization of horizontal lead-cooled fast reactor fuel assembly.