Abstract: The Enping 21-4 oilfield is located in the eastern part of the South China Sea with small reserves. In order to meet the needs of economic and efficient development, two Ultra-deep Extended Reach Wells have been deployed. Aiming at the problems existing in the drilling and completion of two Ultra-deep Extended Reach Wells, such as high well leakage risk, difficult borehole trajectory control, difficult hole cleaning, narrow safe operating density window, and large friction and torque, a series of technologies such as borehole trajectory design and precision control, optimal design of drilling fluid and cement slurry, safe casing running, dynamic monitoring and auxiliary diagnosis technology, anti-wear and drag-reducing completion string have been studied and applied to form key technologies for drilling and completion of Ultra-deep Extended Reach Wells and ensure the successful completion and production of two Ultra-deep Extended Reach Wells. Among them, the Enping 21-4-A1H well has a drilling depth of 9 508.0m, a horizontal displacement of 8 689.0m, and a horizontal-vertical ratio of 4.43, which sets the record for the deepest well depth in China offshore oil and gas fields. The key technologies of drilling and completion of Ultra-deep Extended Reach Wells in Enping 21-4 oilfield not only enrich and perfect the technical system of extended reach well in China, but also provide strong technical support for the efficient development of 10,000-meter marginal oilfield.