Experiments on hydrogen-air vented deflagration in a smooth rectangular tube were conducted with various vent area for different hydrogen concentration. The high-speed schlieren camera was employed to visualize the external flow field. The internal and external overpressures were obtained from five pressure transducers. The effects of vent area for different hydrogen concentration were discussed on the build-up of internal/external overpressures and the evolution of external flow field. Two types of internal overpressure structure, affected by Helmholtz oscillation and external flow field, were observed for different vent coefficient and hydrogen concentration. The increase of hydrogen concentration promotes external mixture reactivity and the external overpressure. The increase of vent coefficient drives the increase of outlet speed of vented gas and flame and generally promotes external overpressure. The higher speed flame provides stronger ignition energy to external combustible gas cloud. For the high hydrogen concentration of 18 vol%, external overpressure increases linearly with vent coefficient. The rise of external overpressure can cause the increase of internal overpressure when it is large enough, particularly in the case of large vent coefficient, and the internal overpressure peak affected by external explosion can even dominate the maximum internal overpressure. (c) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.