Abstract: According to an axial main ventilator with a forced ventilation in a mine, a piece type muffler was installed at the entrance of the air intake duct. After one month of the operation, the cracks were occurred on some board surfaces at the first stage. The paper had a discussion on the main factors affected to the air flow unst ability at the entrance of the impeller. Thus a construction was conducted on the sound absorption board tail and the air intake duct structure and a numerical simulation was conducted on the flow field within the air duct after and before the reconstruction. The results showed that after a muffler was installed at the entrance of air intake duct, under a joint role of the array of Karman vortex street in the wake of the sound absorption board, air flow within the bending air duct and the separated flow at the turning corner of the air duct, the uniformity of the cross sectional main flow velocity distribution at the entrance of the impeller would be increased and the cross sectio nal airflow pressure pulse would be obviously increased and would become the main causes of the blade fatigue failure. The structure improved could obviously increas e the uniformity and stability of the cross sectional air flow within the air duct and at the entrance of the impeller and could ensure the safety of the blade.