For defect mode of 1D doped photonic crystal, a resonant cavity model of 1D doped photonic crystal is established. With coherent superposition principle, transmittance formula and frequency formula of defect mode are derived in I D doped photonic crystal. Coherent superposition theory has advantages of characteristic matrix theory and resonance theory, while it does not have disadvantages of them. Coherent superposition theory is a good approach to study defect mode of 1D doped photonic crystal.