•The critical burst pressure for ignition decreases with tube length increase.•Higher release pressure reduces ignition delay time.•The speed of flame front in the air has a sharp increase.•A strong deflagration is observed and leads to the pressure increase.•The jet flame shows different morphologies at different release stages. Spontaneous ignition and subsequent flame propagation of high-pressure hydrogen release via a tube into air are experimentally investigated using pressure records, flame detection and direct high-speed photographs. The study shows that as the burst pressure increases the likelihood of spontaneous ignition increases and the initial ignition is closer to the burst disk. With the increase of tube length, the possibility of spontaneous ignition increases, while the critical release pressure for spontaneous ignition decreases. It is also found that a strong shock wave generated to trigger the ignition and a long tube to promote the growth of the flame are two key factors for the transition from spontaneous ignition inside the tube to jet flame in the air. After the flame exits from the tube, a flame envelope is formed in the front of the hydrogen jet, which gradually splits into upstream and downstream combustion regions. The upstream flame region propagates forward. However, the downstream flame region moves back toward the tube exit. The flame is then stabilized at the tube exit and gradually grows. Noticeable deflagration events were observed to occur successively in the semi-enclosed space. The deflagration leads to a significant increase of pressure in the chamber. And the overpressure of the deflagration is higher than that of the leading shock wave. Both the overpressures of the leading shock wave and the deflagration increase with the release pressure. A stable jet flame is formed outside the tube subsequent to the deflagration. And different jet flame configurations are observed at different controlling mechanisms of flow.