Abstract: To obtain the law of flame propagation and evaluate the hazard in coal dust explosion, lignite was selected to study the effect of the combustion tube length on the flame propagation of coal dust explosion using a vertical Hartmann tube. A high-speed camera was used to record the flame propagation process. The results show that with the increase of the combustion tube length, the combustion flame gradually slenderized, and the maximum height of the flame front and the maximum velocity continuously increased. The flame propagation velocity increased first and then decreased. When the length of the combustion tube was 300 mm, 600 mm, 750 mm and 900 mm, the maximum height of the flame front reached 649 mm, 808 mm, 1 003 mm and 1 010 mm, respectively, and the maximum flame propagation velocity of the pulverized coal cloud was 5.4 m/s, 12.09 m/s, 15.0 m/s and 18.61 m/s, respectively. The research results can provide theoretical support for ensuring the safe mining of coal.