In order to explain the reason of the defect mode in one-dimensional (1D) doped cycle photonic crystal, a multiple-beam interference model of 1D doped photonic crystal is established. Interference theory of the defect mode in 1D doped photonic crystal is established by interference theory, and the mechanism of the defect mode of 1D doped photonic crystal is explained. The defect modes are studied by interference theory and characteristic matrix method, and their conclusion are the same.