In this paper,numerical calculations of magnetically induced twist effeet in planar nematic liquid crystal are presented.The transmission of polarized light passing through magnetically induced twist distribution in planar nematic liquid crystal is calculated with “4*4”matrix algorithm.On the ex-perimental side,the optical phase retardation method is used to detect the magnetically induced twist effect in planar nematic liquid crystal 5CB(4′-n-penty1-4-cyanobiphenyl) at 27℃.Theoretical cal-culation and experimental results illustrate that polarized light can be used to study the twist effect in thin sample of planar nematic liquid crystal.