Abstract: A study was made on the formula of magnetic viscoelastic body and its seal and compressive strength under magnetic field. The relations were studied among seal and compressive strength, magnetic powder content, strength and distribution form of magnetic field. A new formula was presented to be used for magnetic sealing. It has advantages of acid-resistance, alkali-resistance, insolubility with oil and water and good thermal stability etc., and is suitable for magnetic seal in water/oil wells or other professions.