Abstract: Lead–bismuth eutectic (Pb–Bi) alloy, a liquid heavy metal, has garnered substantial attention as a candidate coolant for next-generation lead-cooled fast reactors (Gen-IV LFRs) and accelerator-driven systems (ADSs) owing to its exceptional nuclear and thermophysical properties. The demanding operational context within which these systems function poses significant challenges to the chemical and mechanical stability of traditional structural materials when in contact with lead–bismuth eutectic (LBE). Addressing the degradation and failure mechanisms of structural and fuel cladding materials in the presence of LBE is of paramount importance for the advancement and application of LFR and ADS technologies. Among various candidate materials, ferrite/martensite (F/M) steel has been considered an ideal candidate for LBE-cooled reactor fuel cladding owing to its excellent mechanical properties, high-temperature mechanical performance, radiation resistance, and lower coefficient of thermal expansion. This study focuses on P91 F/M steel. Dynamic tests were performed for up to 3000 h in uncontrolled oxygen LBE at a relative flow velocity of 5 m·s⁻¹ and temperatures of 350 ℃ and 450 ℃. The surface and cross-sectional corrosion morphologies of the alloy under different temperatures and exposure times were systematically analyzed by scanning electron microscopy. The composition and structural evolution of the oxide layer formed on P91 steel were summarized, and the mechanism of oxide layer spallation was proposed. In addition, electron backscatter diffraction was used to analyze the grain size, stress distribution, and proportion of large-angle grain boundaries in different impact angle regions between LBE and the samples, providing a detailed discussion of the erosion mechanism of the alloy in these regions. At 350 ℃, the oxide layer is a multilayer structure comprising a porous Fe₃O₄ layer, Fe-Cr spinel layer, and inner oxidation zone undergoing a dynamic equilibrium process of “formation–spallation–reformation”. At 450 ℃, the oxidation-corrosion phenomenon is severe, with LBE penetration observed in addition to oxidation. The corrosion characteristics on the sample surface vary significantly across different impact angle regions. The severity of surface damage is ranked as follows: 30° > 90° > −90°. The oxide layer in the 30° angle region completely spalls, with LBE penetrating into the matrix. The 90° angle region shows spalling of the porous outer oxide layer, leaving only the inner oxide layer, with the inner matrix uneroded by LBE. The −90° angle region maintains an intact oxide layer structure that is free from LBE erosion. In this work, the corrosion–erosion behaviors of P91 steel in high-velocity (5 m·s⁻¹) LBE are studied, the formation and spallation mechanisms of the oxide scales are elucidated, and the erosion damage mechanism of the alloy are revealed, providing experimental data and references for the development of structural or cladding materials for China’s fourth-generation nuclear reactors and the study of their corrosion mechanisms in LBE.