The windbox is a compressed air chamber installed directly beneath the fluidized bed boiler grid and is designed to evenly distribute air across the entire cross-section of the air distributor. Due to the large volume of the plenum chambers, it is very difficult to experimentally determine the flow conditions within them. Therefore, the analysis of airflow in these devices is usually performed using computational fluid dynamics tools. The article presents the results of modeling primary air distribution in the plenum chamber of the 700 MWth circulating fluidized bed boiler operating in PGE GiEK S.A. Turów Power Plant Division. Based on the results of calculations performed using the ANSYS Fluent software, the basic problems of the design windbox geometry were identified and a method for improving the uniformity of the gas flow under the grid was proposed. As can be seen from the results obtained, the design geometry of the windbox does not provide the required uniformity of air distribution under the grid.