Abstract: To address the issue of unbalanced air-powder distribution in primary air ducts at the outlet of coal mills in a direct-fired pulverizing system, based on cold and hot-state air velocity balancing tests in primary air ducts, numerical simulations were conducted on the two-phase flow of air and powder in medium-speed coal mills and primary air ducts of a 700 MW supercritical coal-fired power unit. The influence of uneven air-powder distribution inside the coal mill and inherent resistance differences in the ducts on air-powder distribution at the outlet were studied, and the balancing performance of adjustable orifices was investigated. Results show that asymmetric entry of primary air into the coal mill causes unbalanced air-powder distribution in four zones corresponding to the primary air ducts before the separator during actual operation. The maximum relative deviation in air volume distribution is 7.8%, and the maximum relative deviation in powder volume distribution is 23%. When air-powder distribution before the separator is uniform, the maximum relative deviations in hot-state air and powder distribution are 11.26% and 9.12%, respectively. When air-powder distribution is uneven, these deviations become 11.14% and 16.78%, respectively. Inherent resistance differences in the ducts are the primary cause of air volume deviations, while the combined effect of uneven coal powder distribution inside the mill and inherent duct resistance differences leads to powder volume distribution deviations in the outlet primary air ducts. Without an effective powder distributor installed in front of the primary air ducts, adjustable orifices have limited ability to balance powder volume distribution in the outlet ducts, making it difficult to achieve simultaneous balance of air and powder distribution.