Abstract: The Shengli offshore oilfield has a need for long-term, safe and efficient maintenance period for the stratified water injection string in water injection wells.After analyzing the factors affecting the life of the stratified water injection string, this paper proposes a balanced stratified injection string to improve the stress on the string and reduce the peristaltic deformation of the string.To this end, a liquid control method is used to stratify layers to solve the conflicts between layering and well washing.A liquid control packer has been developed to solve the problem of the packer seating in advance and to meet the requirements of well control safety.Through the development of liquid control compression packer and liquid control annulus safety packer key tools, one balanced stratified water injection string was developed, forming a safe and controllable long-term stratified water injection technology.Hydraulic control compression packer adopts hydraulic control method to constantly maintain the optimal pressure state.It can be quickly released during well washing to solve the problem of packer life reduction due to backwashing.Hydraulic control annulus safety packer adapts to a double hydraulic control pipeline, with safety locking mechanism, and thus well washing and safety can be controlled independently.It can not only meet the requirements of large displacement backwashing, but also improve the long-term safety and reliability of water injection string.The balanced stratified water injection string can improve the stress of the pipe string and reduce its creep amplitude.Safe and controllable long-term separated layer water injection technology was used in 101 wells in the Shengli offshore oilfield.The qualified rate of stratified water injection was increased by 16.6%, and the longest maintenance period of water injection string was up to 5 years.Safe and controllable long-term stratified water injection technology not only extends the service life of separated layer water injection string in the Shengli offshore oilfield, but also meets the requirements of safe and efficient water injection.It also meets the requirements of fine stratified water injection in water flooding oilfield.