Explosions are a major type of accident resulting in casualties in the processing industry. Fires frequently occur after a gas explosion. The attenuation rule of the deflagration state beyond the original premixed gas fuel area is the basis for determining the hazard risk once a gas explosion occurs. In this study, a numerical simulation was conducted using a finite element computational code for the fluid dynamics to reveal the attenuation law of the deflagration process of hydrogen-air mixtures in a tube 10 m long with a 0.1 m x 0.1 m sized section with one end closed and the other end open. The combustion characteristics beyond the original premixed hydrogen-air mixture region were examined. The numerical method was verified and its results showed good agreement with the experimental results. It was found that the fuel region can extend and enlarge the area of the flame, and thus a flame beyond the original fuel occupied zone can clearly be seen, which can expand the area of the damage. The explosion state inside the premixed region has a significant effect on the flame area, and a higher explosion strength will shorten the explosion time and reduce the disturbance of the unburned fuel. It was also found that a fuel-rich mixture has a larger flame area beyond the original premixed hydrogen-air mixture. Under all conditions considered in this study, the flame in the hydrogen-air mixture is 1-2 times as long as the one in the hydrogen-air mixture in the initially occupied area.