An adding liquid machine with presettable quantity and a method of adding a right amount of liquid includes a washer extractor, on which a spray apparatus which sprays chemicals on the garment therein is provided; a first measuring apparatus placed on an adding chemicals container for measuring the chemicals contained in the adding chemicals container. The adding chemicals tank is connected fluidly with the spray apparatus through the first chemical pump. The method includes setting a wet pick up ratio and a bath liquid ratio; determining the chemicals amount to be added based on the garment quantity and the set wet pick up; pumping the determined amount of chemicals into the washer extractor, and rotating the inner-drum in the washer extractor to immerse the garment; and extracting surplus chemicals in the clothing. When the extracted surplus chemicals amount reaches the value calculated by the set wet pick up ratio, the extracting will be stopped.