The photonic band gap structure of 1D photonic crystal with an absorptive defect layer was studied by using the transfer matrix method, and presented formulas for the defect mode height and its full width at half maximum (FWHM) under the first-order Taylor′s approximation in the vicinity of central wavelength. Investigations shown that the extinction coefficient,the number of periods and the width of the defect layer had significant effects on the properties of the defect mode.When these parameters increased,the defect mode height decreased quickly. However,the FWHM of the defect mode relied mainly on the number of periods,it decreased quickly with increasing the number of periods, but it changed slightly with the extinction coefficient and the defect layer width.