Abstract: “Deepwater No.1” is the first self-operated deepwater gas field in China, with scattered gas reservoirs and complex sea conditions. Its Phase 2 was confronted with high-temperature and high-pressure (HTHP) geological environments, extensive challenges of drilling and completion technologies and high cost for gas field development. To solve the above problems, the key technologies for drilling and completion design were studied and developed, such as the multi-dimensional optimization of underwater wellhead, multi-factor development well conversion evaluation of exploratory wells, safety drilling duration prediction of extended reach wells in deepwater, and intelligent completion of deepwater gas wells. A series of China made tools and equipment were developed and successfully employed, such as deepwater underwater wellheads, deepwater underwater Christmas trees, deepwater early overflow detection devices, and multi-channel bypass screens. Furthermore, for deepwater development wells, efficient operation technologies were developed, such as large-scale surface batch drilling and inter-well move, safety assurance of the deepwater drilling platform-riser operations in extreme sea conditions. Further, emphasis was placed on the efficient penetration rate improvement in non-target intervals, and integrated completion of upper and lower section of deepwater development wells. The above technologies have effectively contributed to the safe and efficient drilling and completion of “Deep Sea No.1” as well as its second phase. After a review of the advancements of these key technologies, further research direction of deepwater drilling and completion technologies in China is prospected, which is meaningful to guide the development of deep offshore complex oil and gas fields in future.