The invention provides a device and a method for adding a liquid additive. The device comprises a storage tank, a material barrel, a connecting pipe, filters, an input pump, a central controller, a temperature controller, liquid level meters, nozzles, an adding pump, a flow meter and a control valve, wherein, the flow meter is a gear volumetric flow meter; a shell of the flow meter is internally equipped with two rotors which are directly meshed with each other and rotate under the action of pressure difference between an inlet and an outlet of the flow meter; the flow meter continuously discharges fluid which is filled in a metering space between a gear and the shell by means of rotation of the gear; and the flow meter obtains the fluid passing through the same by measuring rotating times of the gear. The device has the advantages of high and stable adding precision and accurate electrical control, thus effectively reducing labor consumption and raw material waste during the production process, and reaching an optimal service condition during the production process.