The flow field of the negative-pressure air knife system of the casting machine was simulated and analyzed by CFD software. The results show that the spoiler and the receiving plate used for uniform airflow have poor effect of uniform airflow，and vortex is generated here，which hinder the rapid discharge of airflow in the vacuum chamber. It affects the uniformity of the film force. Therefore，the spoiler and the receiving plate in the negative-pressure chamber were optimized. The results show that the air mass flow of the optimized negative-pressure chamber is increased from 0.138 kg/s to 0.177 kg/s，an increase of 28.3%，and the circulation of negative-pressure chamber is enhanced. The standard deviation of the velocity distribution in the stable inlet velocity section is reduced from 0.93 to 0.17，and the uniformity of the inlet velocity is improved by 82%，which greatly improve the uniformity of the film force.