<p>3D Concrete Printing (3DCP) is an emerging intelligent construction technology that is reshaping conventional building methods. Among the critical components of 3DCP systems, the extrusion nozzle plays a decisive role in determining the stability and quality of printed layers. In this study, a nozzle with spiral blades, one of the common designs used in 3DCP, was investigated. Although spiral blades can simultaneously transport and mix concrete slurry, they are also prone to issues such as non-uniform mixing and extrusion blockage. To address these challenges, Computational Fluid Dynamics (CFD) simulations based on solid–liquid multiphase flow theory were carried out using Fluent. The internal flow field of a prototype nozzle was modeled and analyzed, focusing on the distribution of cement mortar and water, as well as the mixing and transport characteristics of the multiphase system. The results provide insights into flow uniformity and extrusion performance, offering valuable guidance for optimizing nozzle design to enhance print quality and minimize the risk of blockage.</p>