With the advancement of the “dual carbon” goals, coal-fired power units in China are facing increasingly severe requirements for flexible peak-shaving, and the safety of supercritical boilers under low-load conditions has become a growing concern. To address the safety issues of water-cooled walls during low-load operation, a computational fluid dynamics (CFD) combustion model of a 660 MW tangentially fired once-through pulverized coal boiler was established. This was coupled with a thermo-hydraulic calculation program developed in MATLAB to iteratively solve for the mass flow distribution and wall temperature profiles of spiral water wall circuits. The study was conducted under four typical load conditions: 660, 500, 330, and 231 MW. Results indicate that the wall temperature of spiral water walls exhibits periodic fluctuations, with fluctuation amplitudes significantly increasing as the load decreases. Under the 231 MW condition, the local wall temperature rises to 352 ℃, posing a risk of heat transfer deterioration. A negative correlation between heat absorption and mass flow rate is observed, with increasing flow maldistribution at lower loads potentially causing hydrodynamic instability. The developed coupling model demonstrates high predictive accuracy, with wall temperature deviations within 5% of measured data, and can provide theoretical support for safety assessment and operation optimization under low-load conditions.