Abstract: During the drilling and completion of ultra-deep extended-reach wells (ERWs), technical challenges such as high risk of lost circulation, difficulties in wellbore trajectory control, poor hole cleaning, narrow drilling fluid safety density window, and high friction and torque are encountered. The trajectory design needs a comprehensive analysis and evaluation. In this paper, a model for evaluating the trajectory design scheme of ultra-deep ERWs was established based on the entropy weight method. A total of 25 evaluation indicators in mechanical, hydraulic, and open-hole aspects were selected. Quantitative evaluation and optimization were conducted on the three wellbore trajectory design schemes (constant curvature, catenary, and pseudo-catenary) for the ultra-deep well ERW EP21-X-A1H of the Enping 21-X Oilfield in the eastern South China Sea. The results show that the pseudo-catenary scheme is the most optimal. The operation is carried out strictly in accordance with the designed pseudo-catenary scheme. The total well depth reaches 9 508 meters, and the water-to-depth ratio is 4.43, making it the deepest well in China’s offshore area. The wellbore trajectory closely matches the designed trajectory, and the well quality is excellent. The method for optimizing wellbore trajectory design based on entropy weight theory has been successfully applied in the Well EP21-X-A1H. It provides a new approach for evaluating the wellbore trajectory design of ultra-deep ERWs and has reference value for optimizing drilling technology and risk assessment of ultra-deep ERWs.