<p>As a critical component of the three-phase jet fire monitor head, the nozzle efficiently converts fluid pressure energy into kinetic energy. Optimizing its structural parameters is essential for enhancing jet performance and firefighting efficiency. This study investigates the internal flow field of the nozzle using the VOF method, examining the influence of various structural parameters. The research systematically examines the effects of the nozzle outer wall contraction angle, powder nozzle expansion angle, and length of the nozzle straight section on the internal flow field characteristics. Results indicate that when the nozzle outer wall contraction angle exceeds 25°, the velocity uniformity at the nozzle outlet deteriorates significantly, average turbulent kinetic energy at the outlet of the nozzle increases to 2.37 m²/s², and the pressure drop between the inlet and outlet of the nozzle increases by 1.38%. When the powder nozzle expansion angle exceeds 30°, intensified collision of water flow at the entrance of the straight section further elevates turbulent kinetic energy and pressure drop. The length of the nozzle straight section of 20&#xa0;mm minimizes turbulent kinetic energy to 1.843 m²/s², achieving optimal flow rectification. The findings suggest that rational adjustment of nozzle structural parameters can significantly improve internal flow field characteristics and provide a theoretical basis for structural optimization of the three-phase jet fire monitor.</p>