Oat Avena sativa L. is a widespread cereal plant containing polysaccharides, polyphenols, proteins and saturated fatty acids. Polysaccharides are biologically active substances characterized by expectorant, anti–inflammatory, immunotropic, enterosorbent, hepatoprotective, antioxidant and restorative effects. The extraction of polysaccharides from oat is usually carried out with hot water, ultrasonic radiation, fermentation and deep eutectic solvents. The paper develops and describes a new technique for the aqueous extraction of polysaccharides from the oat plant of milk ripeness. The essence of this method: extraction with cold water from the oat plant in the stage of milk ripeness for 12 hours at room temperature, then boiling the resulting extract together with the herb for 1 minute, infusing for 30 minutes. Then the extract was filtered and used in the work. The analysis of the influence of time and temperature on the quantitative content of polysaccharides from milk ripeness oat with this method of preparing an aqueous extract was carried out. In addition to polysaccharides, phytochemical screening of the aqueous extract of oat grass of milk ripeness for the total content of alkaloids, tannins, glycosides, polyphenols, flavonoids was carried out, and its antioxidant activity was determined. A comparative analysis of different methods of preparing aqueous extracts from milk ripeness oat grass has been done.