In order to research and design high performance optical filters, optical amplifiers, optical attenuators and total reflection mirrors, activation effect of complex defects with two ends symmetry on the optical propagation properties of photonic crystal C(AB)ⁿ(BAB)(BA)ⁿC has been studied by computer simulation. The results show that, when C is real medium defect, there are several discrete transmission peaks with 100% transmittance in the transmission spectrum of photonic crystal with symmetrical structure. When the impurity with gain amplification effect is doped into C to form complex refractive index defect with negative imaginary part, transmittance of each discrete transmission peak in the transmission spectrum will have an amplified gain. Both the imaginary part and the real part of the complex refractive index of the defect layer can modulate the gain amplification of the transmission peak. With the increase of the negative imaginary part or real part of the complex refractive index of the defect layer, the transmission gain magnification increases first, reaches the maximum value and then decreases, and the gain magnification can reach up to 10⁴ orders of magnitude. When the impurity with gain amplification effect is doped into C to form complex refractive index defect with positive imaginary part, transmittance of each discrete transmission peak in the transmission spectrum will have attenuation phenomenon. Both the imaginary part and the real part of the complex refractive index of the defect layer can modulate the attenuation multiple of the transmission peak. With the increase of the positive imaginary part of the complex refractive index of the defect layer, the transmittance of the discrete transmission peaks decreases until the transmittance tends to zero, and total reflection or total absorption occurs. The modulation law of symmetrical complex defects at both ends on the transmission characteristics of photonic crystals can provide theoretical reference for the research and design of new optical amplifiers, filters, attenuators and total reflection mirrors.