Abstract: Conventional intelligent layered water injection systems with hydraulic control in Bohai Oilfield are exposed to problems including low accuracy of allocation, limited layers, failures to meet the application requirement of slim holes and inability of downhole water injection data collection in real time. To solve these problems, research was carried out on the development of digital decoders, stage adjustment optimization of downhole flow control valves, and the design of vortex-shedding flowmeters for high flow rate and layered allocation schemes. In this way, key optimization technologies of intelligent layered water injection with hydraulic control in Bohai Oilfield were formed. After the optimization, six layers could be achieved at the most, and the stages of different flow control valves were raised to 11 and 7 stages, respectively. This means that the requirement of injection wells with 82.6 mm slim holes could be meet, and the real-time monitoring of downhole data could be achieved on the surface. Furthermore, these technologies have been applied to 20 wells in Bohai Oilfield with stable and sound system operation. Good application results can be seen in the following aspects: flexible water choke adjustment, efficient downhole data upload, high test accuracy over 95%, and short average allocation cycle of a single well within a day. The application results show that the technologies can increase the number of injection layers, realizing layered water injection for slim holes and real-time monitoring of downhole injection.