Abstract: Determining the mud flow direction in large-scale horizontal directional drilling projects is crucial for accurately calculating the required circulation pressure, flow rate, and drilling pressure. This paper presents a theoretical model for calculating the flow direction and the aforementioned physical parameters based on the Herschel-Bulkley fluid. The model's effectiveness and applicability are validated through engineering case studies. Variations in required circulation pressure, flow rate, and drilling pressure during pipeline installation are analyzed. Additionally, a parametric analysis is conducted on the effects of rheological properties, eccentricity, height differences between entry and exit points, reaming cycles, and reaming differential on mud flow. The proposed model considers factors such as flow direction and elevation differences, providing an effective analytical tool for analyzing mud flow in large-scale horizontal directional drilling projects.