[Objective] The lower-furnace spiral tube coils of a 660 MW ultra -supercritical coal-fired boiler are subjected to high heat flux and strong thermal deviation conditions. Under such conditions, the flow and heat transfer characteristics of the working fluid inside the tu bes directly affect boiler operation safety. Therefore, it is necessary to clarify the flow and heat transfer behavior of supercritical water in inclined internally ribbed tubes under actual operating conditions, especially in the pseudo -critical region wh ere thermophysical properties vary sharply. [Methods] Based on the actual operating conditions of the boiler, a typical inclined straight tube section in the lower-furnace spiral tube coils was selected as the research object. A three-dimensional numerical model of an inclined internally ribbed tube under half-circumferential heating was established. The shear stress transport (SST) k-ω turbulence model was used to numerically investigate the flow and heat transfer characteristics of supercritical water. Buoyancy and thermal acceleration effects were evaluated using dimensionless criteria. The circumferential wall temperature distribution and the radial distributions of turbulent kinetic energy, turbulent viscosity, wall shear stress,and specific heat capacity were analyzed at three typical cross sections along the heated 2 tube length. The influence of load reduction on heat transfer enhancement in the pseudo -critical region was also discussed. [Results] Under boiler maximum continuous rating (BMCR) load condi tions, the calculated buoyancy and thermal acceleration criteria are at low levels, indicating that their effects on flow and heat transfer are weak. The inner wall temperature shows an obvious non -uniform circumferential distribution, which is significant ly affected by the flow structure induced by the spiral inner ribs. The rib -root temperature is generally higher than the rib-top temperature, and the maximum inner wall temperature appears near the intersection of the rib root and the downstream side. In the pseudo-critical region, the heat transfer coefficient increases obviously, indicating heat transfer enhancement. However, turbulent kinetic energy, turbulent viscosity, and wall shear stress do not increase significantly, which suggests that the enhanc ement is not mainly caused by turbulence intensification. Instead, it is dominated by the sharp increase in specific heat capacity in the near -wall region. With decreasing boiler load, heat transfer enhancement in the pseudo -critical region becomes stronge r, mainly because the near-wall specific heat capacity further increases rather than because of enhanced turbulence, buoyancy, or thermal acceleration effects. [Conclusion] This study reveals the flow and enhanced heat transfer mechanisms of supercritical water in inclined internally ribbed tubes under actual operating conditions, and provides a reference for analyzing water-cooled wall heat transfer characteristics and evaluating the safe operation of ultra-supercritical boilers.