Abstract: With coal-fired units frequently engaging in deep peak shaving, wide-load operation has become the norm, exposing primary air fans to severe stall risks under medium and low load conditions. Targeting Unit #5 of Huadian Zibo Thermal Power Co., Ltd., this study conducted on-site thermal performance tests under three typical load conditions: 300 MW, 225 MW, and 150 MW. The test results show that under the 300 MW full-load condition, the operating parameters of the primary air fan match well with the design values, and the operating point is far from the stall line. However, under low-load conditions, the fan"s operating state deviates significantly from the design range, with the moving blade opening deviation reaching as high as 16%, and the operating point approaching the actual stall line. Analysis indicates that the main cause of the stall is the deviation of the operating point resulting from the forced increase in moving blade opening to meet the air volume demand under low loads. In response, this paper proposes preventive measures such as adding dual-fan flow and header pressure bias logic in the DCS, establishing operating reference value monitoring, and strengthening the maintenance of sealing components. These measures provide engineering reference for the safe and economic operation of primary air fans in similar coal-fired units under deep peak shaving conditions.