Abstract: A hydraulic model of the main pump inlet structure was built to study the influence of the main pump inlet structure of lead-cooled fast reactors (LFRs) on the internal flow and hydraulic performance of the nuclear main pump. Based on numerical calculations and theoretical analysis, the SST k-ω turbulence model was adopted, and ANSYS CFX was employed to conduct transient numerical simulations of the lead-bismuth main pump with an inlet structure, and flow field and performance comparisons were made with the main pump without an inlet structure. The influence of the inlet structure on the internal flow patterns and hydraulic performance of the main pump was investigated. The results show that after adding the inlet structure to the main pump, the shaft power increases, the head increases by 4.5%, and the impeller head in the mainstream area rises. This structure causes non-uniform flow field distribution at the pump inlet, and multiple pairs of vortices and secondary flows are generated at the impeller inlet, thus changing the liquid flow angle at the impeller inlet. The non-uniform flow field ultimately reduces the hydraulic efficiency of the main pump. Therefore, the key point for the engineering application of the nuclear main pump should be the coupling design of the main pump and the inlet structure to both weaken the influence of the inlet structure on the impeller inlet flow field and ensure the hydraulic performance.