The arrangement of downward flowing evaporation heating surface in the furnace can effectively reduce the height of the furnace in the ultrasupercritical circulating fluidized bed boiler. According to the structural parameters of 660 MW ultrasupercritical circulating fluidized bed boiler, the scaled experimental system was established on the basis of the similarity criterion. The distribution of the gas–liquid two-phase flow in the parallel branch pipes of the downward and the upward water-cooled wing-wall was studied under the rated loads of 25%, 50%, and 70% THA. The void fraction distribution, flow distribution of each phase, and dryness distribution in each branch pipe of the downcomers and the risers of the water-cooled wing-wall were measured via the quick-closing valve method and the split-phase measurement method. In the downward water-cooled wing-wall, both the nonuniform coefficient of void fraction and dryness in each branch pipe decreased with the increase in dryness in the inlet header. Under the same average dryness, the nonuniform coefficient of the void fraction increased with the increase in load. In the upward water-cooled wing-wall, under the same load, the uniformity of flow distribution was improved as the dryness increases. Under the same dryness condition, as the boiler load increases, the deviation coefficient of each phase distribution between the parallel pipes increased and the unevenness increased. In the study of flow and heat transfer characteristics of two-phase flow, the void fraction α is a very important intermediate calculation parameter. The correlation formula of the cross-sectional void fraction in the downward water-cooled wing-wall was fitted, and the calculation result was better compared with drift-flux model, with an error of −6% to 6%.