Water injection string suitable for slim hole with small upper-layer displacement and large lower-layer displacement

The invention discloses a water injection string suitable for a slim hole with small upper-layer displacement and large lower-layer displacement. Comprising an oil pipe; the setting device is arranged in the middle of the oil pipe; the Y-shaped flow dividing device is arranged at the lower end of the oil pipe, and a large water injection channel and a small water injection channel are arranged in the Y-shaped flow dividing device; the intelligent water distribution device is connected with the small water injection channel; the fixed water nozzle is connected with the large water injection channel; the upper end of the perforated pipe is connected with the fixed water nozzle and is inserted into the sand control screen pipe, and the middle lower part and the lower end of the perforated pipe are respectively provided with a first insertion seal and a second insertion seal; the positioning bypass is arranged on the upper portion of the perforated pipe and matched with a top sand control packer arranged on the t