Abstract: The steam generator tube rupture (SGTR) accident is one of the most serious design-basis accidents in a lead-bismuth cooled fast reactor (LFR). This results in the injection of high-pressure sub-cooled water from the second loop into the first loop of high-temperature liquid lead-bismuth eutectic (LBE) through the tube rupture at high velocity. The strong interphase heat and mass transfer may trigger a steam explosion, which seriously threatens the structural integrity of the core. In order to investigate the interaction mechanism between a high-pressure sub-cooled water jet and high-temperature LBE, a three-dimensional numerical computational model of the liquid LBE-water-steam multicomponent multiphase flow and the transient heat and mass transfer process of the high-pressure sub-cooled water jet impacting on the high-temperature LBE in a confined space was established. The VOF multiphase flow model, the LES turbulence model, and the Lee phase transition model were employed, considering the role of the covering gas layer in the vessel. The results show that a typical jet can be divided into four zones according to the phase state, i.e., the water-steam transition zone, the multiphase flow zone, the end-water phase zone, and the steam block zone. Jet boiling occurs mainly in the center region of the jet and on both sides of the phase interface. The steam generated by the phase change will entrain the residual water phase migrating along the interface from the end to the top of the jet. The maximum boiling rate under the calculated conditions is usually at the outlet of the nozzle, with a maximum value of 7 090 kg/(m³·s). The pressure of the covering gas layer and the LBE region are positively correlated with both the sub-cooled water temperature and the inlet pressure. The pressure in the LBE region increases gradually as the jet develops, while the migration of steam may cause fluctuations in the pressure of the LBE region. In the 4.4 ms time, the maximum pressures obtained in the covering gas layer and the LBE region are 0.157 MPa and 0.351 MPa, respectively. The results of this paper reveal the jet boiling mechanism and pressure evolution characteristics, providing theoretical support for the safety assessment of the SGTR accident system for LFRs.