The matrix method is employed to analyze the optical activity of liquid crystal, and the matrix presentation of optical rotation is given. The magneto-optical rotation angle of BL-009 nematic liquid crystal is measured using JG-3 type continuous tunable magnetism device and infrared 1350 nm laser. The effect of magnetic field on optical rotation capability of liquid crystal is analyzed in detailed. The threshold of liquid crystal is discussed through experimental testing, and we reached at the conclusion that the rotation direction in liquid crystal is independent of the direction of magnetic field. The obtained result is useful for the design and manufacturing of liquid crystal device.