Properties of transmission spectra of multi-layers consisting of two conjugated photonic crystals are investigated. It is found that, in the case of a small amount of time, the mode density at the interface mode is much larger than that at the band edge. Under certain conditions, the transmission can reach the unity, and the bandwidth can reach the order of picometer. Based on this property, a longitude mode selector of laser consisting of two conjugated photonic crystals made with gain materials is suggested. The effects of the impedance contrast of materials and the refractive index of the environment on the bandwidth are studied.