Under low-load conditions, the water wall of ultrasupercritical boilers often faces severe challenges such as flow distribution imbalance, phase change point migration, and hydrodynamic multiplicity, which can easily induce tube wall overheating and even tube rupture, directly threatening the safety of heating surfaces. To address this, this paper constructs a thermalhydraulic-stress multi-physics coupled simulation model for a 1000 MW ultra-supercritical boiler water wall using the Modelica language. It analyzed three typical operating conditions, BMCR, 75% THA, and 50% THA, to investigate the coupled distribution of hydrodynamic characteristics and stress-strain. The results show that mechanical stress decreases linearly with load reduction, thermal stress peaks due to enhanced phase-change heat transfer, and strain distribution is dominated by thermal expansion. The equivalent stress is controlled by mechanical stress, exhibiting a trend of first decreasing and then increasing with height. The conclusions indicate that the essential causes of increased local failure risk are the coupling effect of flow distribution imbalance and non-uniform heat load under low load. This study provides a theoretical basis for optimization design.