A micro-metering device with a constant flow rate, including a rotary shear valve, a mounting seat, a rotation drive assembly, an injection drive assembly and a controller. The rotary shear valve includes a valve main body and a valve spool. The valve main body is provided with a main flow channel and a plurality of branch flow channels. The valve spool is rotatably arranged inside the valve main body, and is configured to communicate the main flow channel with different branch flow channels. A pressure sensor is provided inside the valve main body, and is communicated with the main flow channel. The pressure sensor is configured for detecting a pressure inside the main flow channel. Whether the switching of the liquid path is correct is determined according to the pressure in the main flow channel.