By means of transfer matrix method,we study the photonic bandgap properties of photonic crystals with two defects.Due to the interaction between two defects,the bandgap structure of one-dimensional photonic crystals with two defects is affected by the optical thickness between two defects and the refractive index of the defects.We defined a correlative coefficient,which can describes interaction between the two defects.And then,we discuss the dependence of correlative coefficient on the septal optical thickness and the defect's refractive index.The photonic crystal's characteristic septal optical thickness is determined by calculation and simulation.