The transmission intension and rotation degree of a BL-009 nematic liquid crystal is measured with magnetism controlled by using JG-3 type continuous tunable magnetism device and infrared 1350 nm laser. The effect of magnetic field on the transmission and rotation light capability of liquid crystal Is detailed analyzed. It can be concluded that the transmission and rotation light effect of liquid crystal are changed with the magnetic field effect. At the same time,the threshold of liquid crystal is discussed.