The core of a small and micro lead-cooled reactor consists of multiple nuclear fuel assemblies. The coolant, after exiting the heat exchanger, flows through the downcomer and enters the lower plenum through a flow distribution plate before entering the core. Due to the structural design of the reactor, the uniformity of flow distribution into the core is affected, resulting in non-uniform distribution of lead flow into the core. This non-uniformity negatively impacts the safety and economics of the reactor. Therefore, studying flow distribution in the reactor is of critical importance for ensuring safety and optimizing thermal–hydraulic performance. In this paper, computational fluid dynamics (CFD) methods were employed to model and mesh the lower plenum, downcomer, and core regions. The porous medium model was used to simplify the structure of the core fuel assemblies. Numerical simulations were conducted using commercial CFD software to analyze the flow characteristics in the lower plenum. Based on the simulation results, the orifice diameter of the flow distribution plate was modified to optimize the uniformity of flow distribution at the core inlet. The numerical results indicate that the flow in the central region of the core is higher than that in the peripheral region. The coolant lead, primarily influenced by gravity, enters the lower plenum through the lower two layers of orifices in the flow distribution plate and flows along the lower wall of the plenum, forming vortices in the lower plenum region. After modifying the structure of the flow distribution plate, the flow distribution factor was optimized to a range of 0.985 to 1.019. This optimization significantly improved the uniformity of flow distribution at the core inlet, thereby enhancing the thermal–hydraulic performance and safety of the reactor.