Abstract: A photonic crystal containing defect layer was composed of a medium (n-type doped GaAs) and b medium (TiO2). Numerical calculations indicate that there are 5 defect mode of 1 transmittance within 3.0-4.5 THz for the photonic crystal. These defect mode have following characteristics: when the doping concentration n of GaAs increases within 1017/cm3 to 1019/cm3, the center, the full width at half maximum (FWHM) and the transmittance of the defect mode remain unchanged, However, if n increased to 1020/cm3, the transmittance of the defect mode began to decline. With increasing angle of incidence, the transmittance of the defect mode remain unchanged, but their center blue-shift, the mobile rate are variable, and FWHM narrows. The central position of the defect mode occurs red shift with increasing the thickness of the media layer a, b or the defect layer c respectively, while both the transmittance and FWHM remain constant. These phenomena provide theoretical guidance for this type of photonic crystal to realize the comb filtering in terahertz frequency range.