The transmission matrix method theory is applied to study the effect of bilateral symmetrical defect C on the transmission spectrum of symmetrical photonic crystal ACmB(AB)n(BA)nBCmA. The result shows that when there is no defect C,transmission spectrum shows a characteristic of mirror symmetry photonic crystal. When the defect C is introduced,the transmission peak becomes wider and moves to higher frequency with the increasing of refractive index nC. The effect of defect periodicity m and optical thickness DC on the transmission spectrum obviously has a characteristic of numerical parity;when m is odd numbers or the optical thickness DC is odd times,there is a wide transmission band appearing in gap center;it narrows down with the increasing of m or DC,and the oscillation on it becomes intense but its center keeps in a stable frequency. When m is even numbers or the DC is even times,there is a narrow defect mode appearing in gap center,and it slowly becomes narrower with the increasing of m or DC,but its position stays the same. These properties can provide theoretical basis for design of width or narrow optical filter devices.