By using the method of optical transfer maxtrix，the band function and the optical transmission properties of 1D three element photonic crystals with an imaginary part of dielectric constant defect are studied.The investigation results show that transmittance peak A and transmittance peak B appear in the photonic band if an real dielectric constant defect is added in the 1D three element photonic crystals.The transmittance peak B is largely affected by the imaginary part of dielectric constant defect.The transmittance peak B is absorbed if the imaginary part of dielectric constant defect is positive.With the increasing of the imaginary，the transmittance gain B is reducing.The transmittance peak B firstly increased then reduced if the imaginary part of dielectric constant defect is negative.However，the transmittance peak A is invariable when the imaginary part of dielectric constant defect is changed.The conclusion is might be used to double channel narrow band filter and optical amplification device synchronously.