In order to obtain resonance theory of 1D doping photonic crystal, a resonant cavity model is set up and the analytical formulas of the defect mode frequency of 1D doping photonic crystal is deduced by resonance conditions of the resonant cavity. The physical mechanism of the defect mode of 1D doping photonic crystal is explained. The use of analytical formulas for the variation, which defect mode frequency with the incident angle and thickness of impurities and refractive index of impurities changes, is studied. Resonance theory results and characteristic matrix method results are compared and their results are the same, and the resonance theory is the right way. The resonance theory to analyze variable relationship is convenient, which makes up the deficiency of the numerical calculation method of 1D photonic crystal.