Abstract: Lead-bismuth reactors, recognized as one of the most promising candidates among fourth-generation reactor systems, can provide substantial support for China’s dual carbon strategy. The lead-bismuth main pump constitutes a critical component in the cooling circuit of such reactors, and its hydraulic performance directly governs the safety and economic efficiency of the reactor plant. To address the complex flow challenges arising from the high density and strong corrosivity of liquid lead-bismuth eutectic (LBE), this study presented a numerical investigation of the internal flow characteristics of an LBE main coolant pump under design conditions. A full three-dimensional model of the pump, comprising the inlet section, impeller, diffuser, and outlet pipe, was established. Transient simulations were performed using the SST k-ω turbulence model within ANSYS Fluent, with LBE specified as the working fluid. The numerical methodology was validated against experimental data obtained with water as the test medium, demonstrating good agreement with a relative error of less than 10% near the design point. The results reveal a substantial pressure difference between the pressure and suction sides of the impeller blades, with low-pressure regions near the leading edge of the suction side prone to flow separation. Within the diffuser zone, pressure increases gradually from the hub to the shroud, and low-pressure regions near the trailing edge indicate the presence of shedding vortices that disturb flow stability. Velocity analysis shows a maximum velocity of 14.4 m/s in the impeller region, which decreases to approximately 4.2 m/s in the diffuser region, confirming effective conversion of kinetic energy. Nevertheless, the outlet diffuser pipe induces flow separation and recirculation, resulting in an estimated head loss of approximately 2%. Pressure pulsation analysis indicates that, under normal operating conditions, the pressure pulsation at the inlet exhibits a sinusoidal waveform and is governed by the periodic rotation of the impeller. Within the diffuser zone, pressure pulsations display pronounced periodicity, with uniform fluctuation amplitudes in the front and middle sections of the diffuser. Owing to the low-velocity separation zone at the trailing edge of the diffuser blades, the fluctuation amplitude of pressure pulsations at the diffuser outlet becomes relatively larger. Along the flow direction, the amplitude of pressure pulsations gradually decreases. Collectively, these findings systematically elucidate the complex flow mechanisms within LBE pumps and offer critical insights for improving hydraulic performance and operational stability in advanced nuclear reactor systems.