Abstract: The approximate formula of the laser induced molecular reorientation angle,refractive index change,threshed field strength and rise time in nematic liquid crystal are obtained by means of the hydrodynamic equation of liquid crystal molecule motion.The dependence of the above parameters on the liquid crystal material parameters,laser intensity and its propagation inside liquid crystal is discussed and calculated.The essential condition which laser can induce molecular reorientation in nematic liquid crystal is given.The essential cause of molecular reorientation in nematic liquid crystal is anisotropy of its molecules have been obtained.