Objective: As the thermal pressure relief device of an onboard hydrogen storage tank is activated by a fire, the released high-pressure hydrogen gas will be ignited to form a jet flame. The large scale and high temperature of the jet flame lead to a potential risk of thermal radiation injury to individuals nearby, including firefighters. This study was based on the bonfire test of a full-size vehicle-mounted hydrogen storage tank, combined with a theoretical system to predict the external thermal radiation of the jet flame and evaluate the danger distance to personnel. Methods: Based on the development process of vehicle fires, the test was structured to include localized and engulfing stages of a bonfire. A Type III hydrogen storage tank with the specifications of 48 L and 70 MPa was used in the test. A pressure sensor and camera were installed to record the internal pressure of the tank and the shape of the jet flame. Based on the real gas state equation and the thermodynamic and fluid characteristics of high-pressure hydrogen, a theoretical calculation framework, which contained the changes to the gas parameters, such as temperature, density, and flow velocity, was obtained. Then, the mass flow rate of hydrogen was estimated by the framework. The heat release rate of the jet flame can be estimated from the combustion heat and mass flow rate. To further determine the radiation flux distribution of the jet flame, this study used the classic single-point-source model. In addition, the prediction accuracy of the model for the external radiation of the jet flame in the near-field (within 3.0 m) has been verified. Finally, the release scenario of the 48 L-70 MPa hydrogen storage tank was cited as an example, and the evolution of the external radiation and thermal dose unit of the jet flame over time at different distances (1.5-5.0 m) was analyzed. The danger distance to personnel was also evaluated in compliance with the injury threshold of the human body under thermal radiation. Results: According to the bonfire test and theoretical calculation of the 48 L-70 MPa hydrogen storage tank: (1) The maximum internal pressure of the tank under full load hydrogen charging conditions in a fire was 77.4 MPa. The maximum flow rate was approximately 0.1 kg/s, with a maximum jet flame length of 4.93 m. (2) The maximum radiation flux of the jet flame at 1.5 m was approximately 13 kW/m², which decayed to 10.2 kW/m² after 20 s. (3) At a distance of 2.0-3.0 m from the flame, it would take approximately 5.0 s to cause first-degree burns to personnel, and at a distance of 5.0 m, it would take more than 20 s. (4) A “risk-free” distance of at least 10.0 m was required to ensure that personnel are exposed to thermal radiation of less than 1.6 kW/m². Conclusions: The results of this study could further improve the theoretical system of jet flame external radiation risk assessment for the emergency release of onboard hydrogen storage tanks that could provide a reference for emergency response in related accident scenarios.