The effects of hydrogen concentration and ignition location on the external explosion were experimentally investigated during vented hydrogen-air deflagrations in a cubic vessel. Hydrogen concentration ranging from 14 vol% to 50 vol% and three ignition locations (front, center, and back ignitions) were focused with the ambient temperature and pressure of 283 K and 100.5 kPa, respectively. The pressure histories and flame behavior inside and outside the vessel were recorded. Experimental results show that Helmholtz oscillation was always found for front ignition. Three overpressure peaks of Popen, Phel and Pvib resulting from the vent rupture, Helmholtz oscillation and flame-acoustic-oscillation were recorded respectively. For center and back ignitions, an additional overpressure peak Pext, generated by the external explosion, was monitored. Popen was the maximum one for front ignition while Pext became the maximum one with increasing hydrogen concentration for center and back ignitions. However, the maximum overpressure for back ignition was significantly larger than those for front and center ignitions. Only one external overpressure peak was recorded for front ignition. However, as hydrogen concentration was larger than 18 vol%, two external overpressure peaks were recorded for center and back ignitions, and the one induced by the external explosion for back ignition was significantly larger than that for center ignition. Moreover, external explosion affected significantly affects the flame evolution outside the vessel.