Supersonic jet characteristics of oxygen lance nozzles have a significant influence on smelting. The three-dimensional model of a 250 t steelmaking converter was established by Fluent software. The jet characteristics of a conventional 6-hole oxygen lance with five holes around and one hole in the center and a post combustion (PC) oxygen lance with a single flow channel were compared and analyzed. The interaction between gas-liquid-slag three phases was simulated by VOF multiphase flow model. The results show that the inflow of the left-side secondary hole flow unit quickly converges into the center hole jet, which has a larger radial cross-sectional area; while the right-side secondary hole flow unit converges into the peripheral 5-hole main flow unit. Due to the supplement of the second hole flow unit, the velocity under the oxygen lance is evenly distributed, and the low-velocity area is reduced, slowing down the decay of the center hole jet, resulting in a large center hole jet velocity. Therefore, the post combustion oxygen lance has a larger impact area and impact depth. Therefore, it can efficiently decarbonize and phosphoresce, giving full play to the secondary combustion effect, and is conducive to slagging. Through the three-phase simulation study of gas-liquid-slag, it can be seen that the cavity profile and interface of steel-slag-gas remain unstable due to the propagation of surface waves, and the slag layer at the furnace wall is calmer with time, indicating that there is no scouring of the furnace wall by either the conventional oxygen lance or the secondary combustion oxygen lance. The maximum impact diameter and impact depth of the secondary combustion oxygen lance is 2461 mm and 358 mm, which are 1.16 times and 1.19 times than the conventional oxygen lance, respectively. The industrial tests have shown that the use of secondary combustion oxygen lances increases the furnace temperature and significantly reduces the oxygen supply time, while the post combustion oxygen lance increases the converter temperature by 27.2°C, and shortens the oxygen supply time by 78 s.