For the analytic study of the defect mode of 1D doped phononic 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 doped phononic crystal is studied. The varition law of defect mode is obtained. The method in freed from the weaknewes of other methods that cannot carry on the analysis of defect mode. The method is valid and accurate method of studying the defect mode of 1D doped phononic crystal.