Abstract: To clarify the mechanism by which nozzle geometry and outlet diameter influence extrusion stability and forming accuracy in direct ink writing (DIW) of high-solid-loading SiC ceramic inks, cylindrical and conical nozzles were investigated. A three-dimensional numerical model of the DIW flow field was established using ANSYS Fluent and validated through experiments. The distributions of pressure, velocity, wall shear stress, and viscosity in the flow field were analyzed, and the forming accuracy of printed structures fabricated with different nozzles was compared. The results show that, compared with the conical nozzle, the cylindrical nozzle exhibits a more uniform flow field distribution and significantly improved stability in terms of velocity, wall shear stress, and viscosity. Owing to these advantages, the cylindrical nozzle produced extruded filaments with better shape retention in experiments. For the four outlet diameters of 0.60 mm, 0.84 mm, 1.20 mm, and 1.55 mm, the forming accuracy of the printed parts improved as the outlet diameter decreased. The cylindrical nozzle achieved the best forming performance at an outlet diameter of 0.60 mm, whereas the conical nozzle still showed flow field distortion and forming defects at the same diameter. These findings reveal the flow field mechanisms associated with nozzle geometry and size, and provide theoretical support for the structural design and process optimization of DIW nozzles.