Hydrogen is considered to be a very promising potential energy carrier due to its excellent characteristics such as abundant resources, high fuel value, clean and renewable. Its safety features greatly influence the potential use. Several safety problems need to be analyzed before using in transportation industry. With the development of the tunnel transportation technology, the safe use of hydrogen in tunnels will receive a lot of research attentions. In this article, the risk associated with hydrogen release from onboard high-pressure vessels and the induced combustion in tunnels were analyzed using the Partially Averaged Navier-Stokes (PANS) turbulence model. The influences of the tunnel ventilation facilities on the hydrogen flow characteristics and the flammable hydrogen cloud sizes were studied. The tunnel layouts were designed according to the subsea tunnel. And a range of longitudinal ventilation conditions had been considered to investigate the hydrogen releases and the sizes of the flammable hydrogen cloud. Then the hydrogen combustion simulation was carried out after the fixed leaking time. The overpressures induced after the ignition of leaking hydrogen were studied. The influences of ventilation and ignition delay time on the overpressure were also investigated. The main aim was to research the phenomena of hydrogen releases and combustion risk inside subsea tunnels, and to lay the foundation of risk assessment methodology developed for hydrogen energy applications on transportation.