The accumulation of corrosion products in lead-cooled fast reactors (LFRs) can cause blockage accidents, leading to localized overheating and posing significant safety risks. To mitigate severe consequences following core shutdown, it is essential to investigate coolant flow and heat transfer behavior under low-power conditions and to optimize coolant distribution. While existing research primarily focuses on flow and heat transfer within fuel assemblies, studies on narrow-slit flow in core bypass regions remain limited. This study conducts numerical simulations to investigate the thermal hydraulic characteristics of bypass narrow-slit flow under natural circulation conditions, considering the impact of fuel assembly blockage. The simulation methodology is first validated against experimental data from Pacio (Germany). Subsequently, the thermal hydraulic performance of the bypass narrow-slit is analyzed at core power levels of 4% and 10%, with blockage cross-sectional areas of 0%, 12.5%, and 25%. Results show that blockage influences not only the peripheral regions but also extends toward the core center. Additionally, the enthalpy rise of coolant at the narrow-slit outlet exceeds that at the core outlet. These findings suggest that a well-distributed bypass narrow-slit flow can enhance core cooling performance and provide an effective supplementary heat removal path during blockage accidents.