With the expansion of new energy integrating into the grid, the demand for deep peak shaving of thermal power units has become increasingly prominent, imposing higher requirements on the wide-load operation capability of power station boilers. The header effect significantly impacts the flow distribution uniformity of parallel tube groups in boiler superheaters. Investigating the header effect under wide-load operation is crucial to ensuring the safe and flexible operation of boilers. Taking the platen superheater of a 135 MW circulating fluidized bed (CFB) boiler as an example, numerical simulations were conducted within the 10%～100% BMCR load range to analyze the flow and pressure distribution characteristics of working fluid in two header configurations with different tube diameters (outlet tubes ?219 mm×14 mm and outlet header ?377 mm×20 mm). The study aimed to investigate the impact of boiler load and tube diameter scale on the header effect of the collection header. The results reveal that as the tube diameter increases, the flow behavior in the header becomes more complex, and the header effect is enhanced. Conversely, as the boiler load decreases, the intensity of vortices and swirl in the header weakens, leading to a reduction in the header effect. A comparison between the simulated static pressure distribution results within the collection header and the hydrodynamic standard calculations in China shows that the standard calculations tend to overestimate the static pressure, with the degree of overestimation increasing as the collection header tube diameter becomes larger.