For analytic studying the defect mode of 1D photonic crystal,the multiple-beam interference method and Bloh theorem are adopted,the analytical formula of defect mode is derived and the defect mode of 1D photonic crystal is studied.The variety regulation of defect mode is obtained.The method avoided the characteristic matrix methods and the F-P methods weakness that can′t carry on analysis defect mode.The method is valid and accurate methods that study the defect mode of 1D photonic crystal.