Venting of hydrogen-air deflagrations is a complex process, and many issues remain to be investigated. In order to analyze the mechanism and influence factors of the overpressure development during vented hydrogen deflagrations, a commercial code FLACS was used and the capability of the code was validated by previous experimental data. Based on the experimental and numerical results, the effect of concentration, ignition location and vent area on the vented overpressure was discussed in detail. It was confirmed that in the condition of the large vent area, three overpressure peaks are formed at the moments of the vent failure, the external explosion and the occurrence of the maximum flame surface area in the vessel, which are marked as P-burst P-ext and P-mfa. The overpressure peak P-ext is corresponded to the formation of the external pressure caused by the external explosion, and peaks P-ext in BWI conditions are larger than those in CI conditions. The relationship between overpressure and vent area match the power law with negative exponent, while the larger vent area may lead to the stronger effect of the external explosion on the internal overpressure. Moreover, the differences in magnitude between P-ext and P-mfa were discussed. (C) 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.