Abstract: With the continuous deepening of green power production and supply and the increasing proportion of new energy power generation, thermal power units are transitioning to a role that emphasizes both basic security and system regulation. Supercritical circulating fluidized bed (CFB) boilers, due to their unique flexibility advantages, need to undertake deep peak regulation tasks. Therefore, based on the Computational Particle Flow Dynamics (CPFD) method, a 350 MW supercritical CFB boiler was taken as the research object, and detailed simulation studies on the gas-solid flow, combustion, and heat transfer characteristics in the furnace were carried out under wide load conditions ranging from 30% to 90%. The three-dimensional particle mass concentration distribution, the flow uniformity of parallel multi-loop systems, the three-dimensional temperature distribution, and the distribution law of heat transfer coefficients on water-wall heating surfaces under different loads were specifically analyzed, and the NO_x formation and emission characteristics were discussed. The results show that as the boiler load increases, the circulating flow rate increases, the particle mass concentration in the upper part of the furnace increases, and the axial S-shaped solid mass concentration distribution tends to be gentler. At the same time, the temperature difference between the upper and lower parts of the furnace decreases, the uniformity of temperature distribution is enhanced, and the axial attenuation rate of the heat transfer coefficient decreases. In the depth direction of the furnace, the particle mass concentration at the rear wall is higher than that at the front wall, the temperature is lower than that at the front wall, and the heat transfer coefficient is higher than that at the front wall. Moreover, the higher the load, the greater the mass concentration difference between the side-wall and central regions. In the width direction, the distributions of particle mass concentration and temperature show obvious symmetry, but in the middle and upper parts of the furnace, affected by suspended heating surfaces such as water-cooling screens and superheaters, each parameter shows a wave-like distribution. Notably, the gas-solid mass flow rate of the middle separator is lower than that of the side separators, and the “one-to-two” structure of the return valve leads to a reduction in the flow uniformity of the parallel loops. Under low-load conditions, the uniformity of particle flow among the parallel multi-loops is improved, but the deviation of heat transfer coefficients on each wall of the water-wall further increases. In addition, along the height direction of the furnace, the NO_x mass concentration shows a trend of first increasing and then decreasing. As the load decreases, the original NO_x emission mass concentration shows a characteristic of first decreasing and then increasing, with the inflection point occurring at approximately 50% load.