A series of experiments were carried out in a 27m 3 cubic chamber with different vent areas to study the external pressure during vented lean hydrogen deflagrations. Uniform hydrogen -air mixtures with concentrations ranging from 15 % to 21 % and stratified mixtures with maximum concentrations equalled to average values of uniform mixtures were tested. Different layouts of obstructions were used in the experiments to achieve different volumetric blockage ratios. Experimental data from vented deflagration tests were presented, and the effect of hydrogen concentration, vent area, non -homogeneous mixtures and obstacles on the external pressure during vented deflagrations was investigated. The results show that the external explosion has an effective influence on the internal pressure by several ways. The external pressure increases with the hydrogen concentration inside the chamber, and the maximum external overpressure with back ignition is higher than that with center ignition. Larger vent area leads to a higher external pressure measured at the position far from the vent, but an opposite trend is found at the position close to the vent due to the higher velocity of jet flame corresponded to the smaller vent. As the volumetric blockage ratio increases, the external overpressure increases accordingly, and the difference in magnitude between back ignition and center ignition becomes more pronounced. However, for the non -homogenous hydrogen deflagration, whose external pressure is mainly governed by the maximum hydrogen concentration inside the chamber, only when the volumetric blockage ratio inside the chamber is large enough, the obstruction can have a significant influence on the external pressures, but its effect is much stronger for the homogenous deflagration.