To address the measurement deviation of primary air flow rate at vertical-shaft mill entrance in direct-fired system for coal-fired power plant, the computational fluid dynamics (CFD) software is used to simulate the flow field in primary air duct. The computed flow rate is compared with the measured values, which indicates that the small aperture of symmetrical damper will produce flat fluidic wall-attachment phenomena and makes downstream flow field uncertain and badly stratified, so random arrangement of measuring points cannot reflect the actual flow reasonably. It is proposed to choose the appropriate position for flow measurement based on the CFD simulated results.