Abstract: To address the issue of large steam temperature deviation at the vertical water-wall outlet of a 1 000 MW ultra-supercritical double-tangential circular boiler under low load operation, the water wall was equivalent to a flow network system, and a mathematical model for hydrodynamic calculation was developed. Using the measurement data of the experimental furnace under 400 MW load, the actual heat absorption deviation distribution in the furnace was repeatedly back-calculated and iteratively calculated, and the precision of the model was validated. Through analysis, the asynchronous dry-wet state transition during low-load operation was revealed to be the cause of the outlet steam temperature deviation, and relevant operation countermeasures were proposed. Two throttle adjustment schemes were designed, hydrodynamic characteristics after throttle orifice adjustment were calculated, and the flow instability was checked. The results show that with the two adjustment schemes, the outlet steam temperature is more uniform, the outlet steam temperature deviation is reduced, and no flow instability occurs. After adopting the second throttle adjustment scheme, the overall outlet steam temperature of the four walls is more uniform, and the improvement effect on the outlet steam temperature deviation of the four walls is better.