Abstract: The phase-change characteristics of liquid water jets are of great significance for the efficient water-fuel combustion in water ramjet engines. To investigate the dispersion and phase-change behavior of liquid water jets under high-temperature incoming gas, a three-dimensional transient multiphase-flow computational model was established by adopting the VOF multiphase-flow model coupled with the Lee phase-change model. The effects of nozzle configuration (straight, convergent, and convergent-divergent), incoming gas temperature (600 K, 800 K, and 1000 K), and the distance between the water nozzle outlet and the gas inlet (80 mm, 100 mm, and 150 mm) on water-jet characteristics were simulated and analyzed. The results show that the convergent nozzle generates a more stable and concentrated water jet. Through the jet aggregation effect, it suppresses radial dispersion during the phase-change process and achieves more uniform mixing between phase-change-generated water vapor and gas flow. As the incoming gas temperature rises from 600 K to 1 000 K, the water phase-change rate accelerates, the dispersion of both liquid water and water vapor is significantly enhanced, and the radial distribution uniformity of the phase-change-produced water vapor is markedly improved.The diffusion range expands from 22 mm to 35 mm, the mixing performance between the multiphase water jet and gas flow is improved, and the proportion of high-mixing-degree regions increases from 11% to 23%. There exists an optimal distance between the jet outlet and the gas inlet. At a distance of 100 mm, compared with distances of 80 mm and 150 mm, the water jet achieves superior dispersion and a moderate heat-exchange duration, leading to more effective mixing between the phase-change-generated water vapor and the high-temperature gas. The convergent nozzle controls the phase-change boundary mainly by preserving jet-core momentum, whereas elevated incoming-gas temperature accelerates water vaporization rate via interphase heat transfer. A proper injection distance facilitates the coupling of penetration, vaporization, and entrainment diffusion, thereby strengthening the phase-change effect of the water jet.