Accidental release of high-pressure hydrogen can result in self-ignition, non-premixed jet flame and high overpressure, which will create potential security risks to people, buildings and equipment. In this study, pressure dynamics and flame induced by the self-combustible hydrogen flowing into the unconfined space were experimentally studied. The entire process was characterized by pressure sensors and cameras. Results show that the velocity of the hydrogen jet increases first and then decreases after it flows out of the tube. Its overpressure decays rapidly and stabilizes quickly. The dramatic changes for the flow field parameters in the near-field region can cause the self-ignition region to extinguish first and then reignite. And the overpressure in the near-field region, caused by the self-ignition jet flowing out of the tube, is lower in the unconfined space than that in the semi-confined space. In addition, axial and radial variations for the jet flame are characterized by phased development, which can be divided into three stages based on features of flame morphology and temperature distribution. Among them, a typical tadpole-like flame is formed. Its head with large size shows asymmetry, develops downward deflection and eventually separates from the flame body which are all affected by asymmetric large-scale vortices. Besides, the re-ignition of the jet can be induced during the nitrogen purge process, which is much more dangerous in real accident scenarios. (c) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.