Based on the theory of transfer matrix method and computer simulation,the modulation law of the number of elementary medium on the optical transmission characteristics of photonic crystal has been studied.The results show that there is a main forbidden band with wide bandwidth in the transmission spectrum,whether they are standard or symmetric structures.Moreover,bandwidth of the main forbidden band of photonic crystals with different number of elementary media is different.For standard periodic photonic crystals(AB)m,(ABC)m and(ABCD)m,there is no transmission peak in the main forbidden band.However,with the increase of the number of elementary media,the bandwidth of the main forbidden band also changes,and there are several discrete transmission peaks on both sides of the main forbidden band.For symmetric photonic crystals(AB)m(BA)m,(ABC)m(CBA)mand(ABCD)m(DCBA)m,there is a constant narrow transmission peak with 100 % transmittance in the main forbidden band.With the increase of the number of basic medium,the number of narrow transmission peaks and their transmittance remain unchanged,but the bandwidth of the main forbidden band and its narrow transmission peak will become narrower.At the same time,there are several discrete transmission peaks on both sides of the main forbidden band,and the number of discrete transmission peaks is more than that of standard periodic structure photonic crystal.The modulation law of the transmission band spectrum of photonic crystals with standard periodic structure and symmetric structure by the number of elementary medium can provide theoretical guidance for the research and design of new optical filters,optical switches and other devices.