Background: Since ancient times, humans have used medicinal plants for medicinal purposes, and they have been recognized as beneficial to health. The basis of the treatment of these plants is phytochemical substances and in modern times, with the development of science and technology, the substances contained in these plants are extracted and medicinal substances and drugs are produced to treat many diseases. There are more than 800 medicinal plants in Mongolia, and their chemical structure and properties are studied in detail, biological activity and main features are discovered, and new medicines are produced. From the Thalictrum squarrosum plant, 25 compounds have been identified, such as triterpenoids with 17 carbon atoms in the cycloartane type ring, saponins, oleic acid glycosides, phytosterols and their glycosides etc., and the structures of 7 new compounds have been determined and the research on phenolic compounds and antioxidant activity has been limited, which is the basis for this research. Aim: Determination of total phenolic content and antioxidant activity of Thalictrum squarrosum. Materials and Methods: Total phenolic compounds were determined spectrophotometrically using Folin-Chicolte color reagent in 1.0 raw material of the Thalictrum squarrosum. Antioxidant activity was determined by DPPH, FRAB and ABTS in sub-extracts of 4 groups. Results: The total phenolic compounds of Thalictrum squarrosum were determined to be 1.9±0.001%. In the study of biological activity of Thalictrum squarrosum plants, the antioxidant activity of 4 groups of sub-extracts was tested by DPPH method, and the IC50 of total alkaloids (pH 9-10) was 201.58±0.1 μg/ml, ethyl acetate sub-extract 94.34±0.66 μg/ml, aqueous extract 824.18±0.08 μg/ml, and butanol sub-extract 128.75±0.58 μg/ml. When tested by ABTS method, the IC50 of total alkaloids (pH 9-10) was 35.83±0.44 μg/ml, ethyl acetate sub-extract 90.45±0.62 μg/ml, aqueous extract 104.19±0.93 μg/ml, dichloromethane sub-extract 156.44±0.48 μg/ml, and butanol sub-extract 170.03±0.61 μg/ml. When tested for antioxidant activity at 800 μg/ml by the FRAB method, the ethyl acetate sub-extract was 8946.16±14.79 μg/ml, the dichloromethane sub-extract was 1670.12±39.28 μg/ml, the butanol sub-extract was 4863.97±25.98 μg/ml, and the total alkaloids (pH 9-10) were 4897.99±58.12 μg/ml. Conclusion The ethyl acetate extract of the plant Thalictrum squarrosum has been found to be highly antioxidant active and total phenolic compound was 1.9±0.001%.