Abstract: The blowing of oxygen at supersonic velocity through nozzles is a fundamental method and key technology for basic oxygen furnace process used in the steelmaking process. During the process, the high-speed oxygen jets penetrate the liquid slag leading to the formation of the impaction cavity on the surface of the molten bath. Further, the dynamic energy and mass transfer would occur at the three-phase (oxygen–liquid slag–molten steel) region. As a result, the impurity elements are removed, the temperature of molten bath is controlled, and the solid slag is melted faster. Moreover, many complex wave structures are formed in the traditional Laval nozzle depending on its gas flow field, resulting in suppression of the initial stirring ability of the oxygen jet. However, the new Laval nozzle designed by the characteristic-line method can solve this problem. Additionally, Mach number, dynamic pressure, and entrainment phenomenon of both traditional and new Laval nozzle structures were tested using various oxygen flow rates at the high-temperature ambition environment. The results prove that the new Laval nozzle structure prolongs the velocity core length of oxygen jet, increases the molten bath stirring effect, and improves the mass transfer process.