Abstract: In order to solve the problems of replacing the master valves beneath the Christmas trees of producers and injectors with traditional methods, such as long construction period, high operating cost and reservoir damage, a series of studies were conducted on Christmas tree replacement under pressure, including tubing plugging, wellhead sealing, pulling and running the plugging tools. A tubing plug with pressure-bearing 35 MPa, an overshot with tensile strength 240 kN, a two-way hydraulic cylinder with maximum tensile force 157 kN and maximum thrust 227 kN, and a lubricator with burst strength 70 MPa and sealing pressure 35 MPa have been manufactured. The technological process was developed to replace the Christmas tree under pressure, and tested on site at Well ZH4×2 in Dagang Oilfield, it can shorten the period of the well cycle by about 20 days, recover water injection in advance about 400 m3, and achieve cost savings of 78, 000 Yuan or so per well, compared with conventional technologies, thus effectively avoid reservoir damage and keep injection-production balance. Application results showed that the technology of replacing the Christmas tree under pressure can solve such problems as long construction period, high operating cost and reservoir damage, etc.